Once in a while, as I stand on the sidelines watching a game, before or after one of my own, I'll hear a spectator yell, "He's offside!!!". It's always when their team is defending an attack. Inevitably I'll suggest to that spectator........"I wouldn't do that if I were you. What do you think will happen if YOUR defender hears that, stops or just slows up for a few strides? That attacker now has a breakaway scoring attempt, as YOUR defender mistakenly removed himself from the play. Do you REALLY want that to happen?"
As far as defenders raising their arms for an offside call.......it's "Gamesmanship", in a mild form, yes. But it's still an effort to influence the calls that the officials are there to make. Same as Coaches pointing the direction of a throw, or yelling for a whistle / foul. When they happen to guess correctly, and we then make the call.......it CAN make it look like we're calling what THEY want, not what we have decided.
Bottom Line: Coaches should coach......players play......spectators cheer & voice their support for their team. PERIOD. "Stay in your assigned lane".