Interesting that May is the leading month. I believe I've read someplace that May has the *least* number of babies born in that month. August was the highest IIRC.
I prefer Sept/Oct/Nov. Biggest reason is for school age cut off. I like my kids being the oldest in their classes, and our cut off is Sept 1st.
I liked that we were just hitting the 'winter' months (we live in TX, so not much winter really) cause I try to keep my babies out of public for a while anyway, and that's usually the easiest time of year to do it.
Of course, now that I've actually got school aged children, a brand new baby born just before winter might be a bad thing in terms of germ exposure.
For school cut off reasons, I personally wouldn't want a baby between May-August. Of course, typing that out, I'll end up with an oops due somewhere during that time frame.

I always swore I wouldn't have a December baby, but my fourth was due Dec 18th. I ended up having her at the end of November, but still...