I'd always heard to spay around six months or before a female's first heat.

But then I read this article the groomer put up in the breakroom and you know it really kind of makes sense to me.

http://www.caninesports.com/SpayNeuter.html

Grant it, so long as you're a RESPONSIBLE owner, it shouldn't be a problem for a year or so until you get them fixed. Then again for every responsible owner there are 50 irresponsible ones...