As Tuvok is Vulcan, I would think that he'd view basically everything in light of Starfleet guidelines, including how subordinates address him. However, he is apparently okay with Harry Kim and Tom Paris, for example, calling him by his first name. This is the case even though the two are of lower rank and sometimes obnoxious.
Is there a canon reason why Tuvok doesn't ride them (and other subordinates) for not addressing him as "Sir" or "Lieutenant"?