
Yes it is sa myth and breeding is a very serious subject too often entered itno lightly. After all you are bringing into the word the next generation for good or ill. The whole purpose of breeedign shoudl be to improve the breed. To do this you need to ahve a lot of knowledge of your breed and the individuals in your bitches and stud dogs pedigrees, then a good dollop of Luck.
Spaying is best done half way between seasons, if she has had more than one you can guess better, but if she has previously gone 6 months or more then at least two months before her next one is due is best. I would tend to go with 3 months after the last season started in case she is one of thsoe who comes in sooner. I deally you want no signs of swollen mammaries, but not too close to the next season as the increased blood supply to the uterus makes bleeding during or after the operation more of a risk.