I've been following this thread with so much interest, I've forgotten to chime in myself!
We're in a similar situation in that DS, 3.5, was born 2.5 weeks after the cutoff, and we are also considering early entry.
The funny thing is that they have been playing around with the cutoffs so much in my state (we live in Europe, not North America) that for two out of the last four years, DS would have been eligible, for two he wouldn't. Do you follow me? Ha!
Cutoff used to be 6/30 (for 1st grade, K is traditionally a pull-out program in pre-school, with entry being determined accordingly). There has always been a rule in place that children born until the end of the year could be entered on request and agreement of the principal (redshirting works the same way, so it's not completely up to parents, but I haven't heard of any child being forced into entering). I have read that 50% of July-born children, 30 % of August-born children and 10% of Sept-born children used to be entered early, with later months negligible, so it was a very common thing as long as children were 6 when schools started mid-September.
Because they were worried that a growing trend towards redshirting and a perhaps overused retention policy were making kids too old for the developmental expectations of their grade and for entry into university/the workforce, they started to move the cutoff ahead to 9/30 three years ago, planinng to keep moving it for another month every year until they finished up with 12/31. In accordance, they flipped the rule so that you could redshirt children born after 9/30, but before that year's relevant cutoff, on simple request without anyone having to agree.
So three years ago, our October-born child would have needed a request and agreement for early entry. The following two years he would have been a regular first grader and if the law had been kept in place, this year as well. Enter parents: turns out that as soon as they moved beyond the September cutoff, a whopping 70% of October- and November-borns were redshirted by their parents. I think that there is a gut feeling in place that five isn't old enough for first grade, period!, which they hadn't taken into account. So they announced that they were moving the cutoff back to September, with early entry for Oct-Dec-borns on request and agreement of the principal.
What a lottery, eh? I know it's always been the same curriculum, the same classrooms, the same teachers, the same expectations, but what a difference this may have made for children whose parents weren't savvy enough to make sure to make the best choice for their child. It is also annoying for parents like us who put our DS' name down for a couple of private schools three years ago in the expectation that eligibility for entry in that particular year was a given, as private schools follow the law for public schools. Waiting lists for these schools fill up quickly and it may mean that unless we all agree on early entry (I think that apart from wanting it ourselves, we need the agreement of the private school and the agreement of the principal of the public school we're zoned for, if that makes sense) he would not have a place.
But that's all to make you laugh or shake your heads or both (and maybe sigh in relief that where you live isn't so bad). I would have considered early entry anyway, for the following reasons (which sound a lot like the OP's considerations for your daughter):
DS is academically precocious. He isn't reading yet, but with the trajectory he appears to be on, I expect him to read well before he is regularly eligible for K. (I might be wrong, of course). If we do not enter him early, we may have to grade skip later, and while I am not opposed to a grade skip as such, having been skipped myself, I know it is much harder to pull off successfully (my own grade skip was certainly handled very badly, but I never regretted getting out of grade school a year early).
We have entered him early (as in not-quite-three in September) for pre-school, and being with older kids ( 3-6 mixed-age classes) and doing their stuff worked much better than daycare - he was suddenly able to handle a whole lot of issues (noise sensitivy, separation anxiety, poor peer interaction etc.) so much better with being appropriately challenged on the cognitive front. I tremble at the thought of keeping him in pre-school as one of the oldest until he's almost 7 - he's playing with the 5-and 6-year olds now.
We've entered him in music class a full year early, and it works great. He loves it, participates very well, does not act out, and he is not the most fidgety by far! (Attention issues being the ones I'd be worried about).
I am not worried about his fine motor skills, which I am told can be a big issue with boys entered early. His drawings are amazingly intricate now, and he can write his fairly long name, handle scissors etc.
And yes, he is big. People routinely take him for a year older (part looks, part size, part verbal skills). I am also planning to get him involved in some well-regarded sport on the vague notion that being athletic helps a lot with peer acceptance (not so easy because he is slightly hypotonic. I hear that karate and swimming are recommended). Thus, being behind in gross motor skills is another issue of worry, but not one that would make me reconsider, because it will be an issue no matter which year he is being entered.
Edited because I figured this post wasn't long enough yet:
Both DH and I were among the earliest to go through puberty, despite being young for grade/a year younger. Physically maturing earlier than almost everyone else wasn't so hot - I am glad I wasn't quite the first in my grade. It's such a toss-up, not something I'd base a decision for a kindergartner on. I wasn't ready for dating when my classmates were, so I just didn't. (I wasn't ready a year later, either, so just had to deal). And I got a lot of lifts when everyone else got their licenses, so as soon as I had mine, my parents let me have the car a lot at first for payback. It wasn't an issue.
A final note to the OP:
You have said that you do not think your DD is academically precocious because she is not reading or doing arithmetics now - but if she were doing that, she'd have mastered K skills now. She may still be very advanced. I recommend checking out the PBS development tracker for stuff an average 3-year-old should be doing and comparing it to your daughter. You may be surprised.