Truth or myth about spayed dogs.

I've always heard that dogs that have puppies than spayed make better dogs than spayed and not having puppies.I want to have mine spayed,she just turned four months.Is this just a myth,or is there any truth to it? Also,when is the right age for dogs,or pups to be spayed? I've always heard 6 months.I heard that dogs that don't have puppies and spayed,it changes their temperment.I don't want that to happen because she has such a sweet temperment.