A player at our club who has bounced between 3.5 and 4.0 foot faults on 100% (no exaggeration) of his serves and to my knowledge no one at the club has ever talked to him about it nor has an opponent in a league match called one on him.
He has a very slow, deliberate serve warm up and hits a pretty hard flat serve but on every one he starts with his toes on the line and slides his front foot forward where his toes end up on top of or just slightly past the line before making contact.
I remember watching him in a social match at the club and several people commenting on his foot faults but no one offered to speak to him about it. I partner with him as much as any other player in league play and I've not had the stones to talk to him about it either. I think perhaps his teammates are hoping some opponent will call one on him eventually and let him know. I believe he doesn't do it intentionally but I can't say that he doesn't gain a slight advantage by doing it.
It just seems like such an awkward conversation to have to tell a tennis player he is a habitual foot faulter, like telling someone you know and like that they have bad breath. Who wants to have that conversation?
When should you serve and volley in doubles? Only when you want to win.