Oh great, another chance for the non's to say 'there are no gentlemen flossers blah blah blah'.
Anyways, I think it depends on the 2 individuals and how you have to cast at your particular spot. If someone steps into my space and it changes what I have to do to cast, I'll say something. But if they step in and adjust their casting to suit me, then it usually works out. Where I go, I cast left to right due to the makeup of the bank behind me (so do the others), but potentially someone could come and touch my right shoulder and cast overhead from the right without problem. Years ago I fished peg and remember it to be an overhead cast on the right side, so if they step in on your left, it would really depend on how they cast. All scenarios are different.
Sometimes I like it when someone buds in early. Just get it over with and make a friend, and not have to worry about it again, cause at some point it'll likely happen.
Rob