To be honest, no offense but it's kind of a stupid question. looking at your opponent's screen is always cheating. I guess it's a strategy in a way but it's simply lazy and defeats the purpose of a competition.
My friends and I will play halo together sometimes. And I try really hard not too, and if I have a sniper, and I accidently see them sneaking up behind me or something, I wont move until I woulud move if I hadnt seen them. The other 2 I play with, idk what they do, they might peek. I know they do sometimes, and sometimes I do too... Like when I have a sniper on there head, and all of a sudden they move where I cant see them..