My son, who just turned 5, is at the level where he can read Harry Potter now. I am not bragging-- at all-- and attribute this in part to my helping him early on. I did this because I wanted to take as much advantage of what I think of as the early "Montessori period" of explosive growth as possible.
On the other hand, I assign a great deal of importance to kids being independent, and remember fondly teaching myself to read as a kid. In a sense, I've robbed my son of that joy and feeling of accomplishment, though he can get it in other ways and though he certainly has had a big hand in learning to read too. In addition, I think that a lot of early readers just read stuff for fun (witness Harry Potter), not advanced physics textbooks or something.
I guess that a correct answer to the specific question of whether it's best to wait for a kid to read on their own or to teach them depends in part on unanswerable questions (how to stack the feeling of achievement and self-realization gained, against any learning/achievement advantage conferred by earlier reading?). It also depends, I guess, in part on knowledge of how many kids would read abnormally late, to the point of serious disadvantage, if left to their own devices.
In any event I don't think six is too late to learn to read; I'm just musing. I am thrilled for you and your daughter. I hope that her sense of accomplishment and excitement carry her far, as they very well may.