It can depend. A while back I used default for certain weapons, +5 max for close range and rollers, and -3, -4, even -5 for chargers and eliters. At some point I decided to just get used to using EVERYTHING on +5 because I'm often changing weapons in private battles between chargers, splooshes, and anything else, and while I can change weps and gear between matches, you can't change sensitivity without leaving the lobby. So I had to pick a sensitivity and stay with it. Now, even sniping just feels normal on +5, though I admit since I'm out of practice with eliter since they nerfed it to oblivion, I'd probably be better off reducing it on the very rare instances I still pick up an eliter.
IMO it's easier to learn sniping at very low sensitivity, but with practice you can do it as effectively at any sensitivity.