I have heard some pretty screwy stuff from people who do not care for any type of spay/neuter. But I think the best reason to wait is to get let the uterine horns, etc get back to normal.
The way the canine body works is whether the bitch is bred or not, her uterine horns under go the same progesterone blast/hormone changes. If you wait two months after the heat cycle, that would be when the puppies would be born -- they are born 9 weeks (63 days) after ovulation, which usually happens around the middle of the heat cycle. Anyhow, if you are counting from when she finished her heat cycle, the pregnancy would be completed by eight weeks.
Bitches are interesting. They blow coat twice a year. Normally at the time when the puppies would be weaning, whether or not she has been bred. So their hormones, are working and effecting them throughout the cycle.
I would wait 3 months after the heat cycle if it was me. That would put her right between, and her stuff would be back to normal.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC