Indy's almost 8 months old now, and I still haven't made the appointment to spay her. It's just so confusing about when is the right time. Personally, I'd prefer to just have it done ASAP and be done with it, no going through a heat cycle, no having to keep her home when she'd much rather be going on adventures in the park and coming along to agility trials, but I certainly don't want to cause her any harm, or damage her structure in any way. It's much easier with males, just wait until their growth plates are closed, but it's trickier and more complicated with females.
www.caninesports.com/SpayNeuter.html A good read on the subject
Rudy was spayed at about 7 1/2 months, the others were being shown, so they were all over a year old by the time they were spayed/neutered. I really can't say if it made a difference one way or the other, since they're from such drastically different lines.