You'll probably enjoy The Biodeterminist's Guide to Parenting[1] by Scott Alexander[2]. It's a good overview of environmental effects on cognition. There's a section on birth month, and the answer is: It's a trade-off. If you're sending your child to public school, August is best. This makes them the oldest in their grade, giving them advantages that compound throughout school. For private school, February/March might be best, as winter babies have slightly higher intelligence and social skills.

1. http://squid314.livejournal.com/346391.html

2. You may have heard of him. Scott is a resident psychologist and writer of Slate Star Codex (http://slatestarcodex.com/).