I have put this age-old question to the test myself and I would have to agree that breeding a bitch doesn't affect a thing. Here is what I did:

I had a dog named Pretty Boy, who was a 35 lb 1xW in 2:42 and a 1xL(G) in 1:10. Pretty Boy couldn't bust a grape, but he was game as a dog can get. I used him to start an 11 month old puppy named Icon. (Yes, I normally don't start dogs this young, but Icon was so intense and so destructive on the chain, I figured he needed to release some energy.) Anyway, Icon was only 31 lb.

Well, I had been breeding Pretty Boy all week long when I did the roll, and the 11-month old PUPPY Icon mopped-up the floor with Pretty Boy. I figured "it had to be" because I was breeding Pretty Boy. Well, check this out

9 months later, Icon proved to be an Ace-level head dog, and HE was now breeding bitches ... and I had bred HIM all week long to 3 different bitches. So I got to thinking, "I wonder if this will affect his performance?", and so I again put him with Pretty Boy (who was the one rested now, while Icon was the one being bred). And guess what happened?

Once again, Icon mopped-up the floor with Pretty Boy, and the conclusion I drew was that the better dog will win, whether you breed him or not

Jack