Recently I complained about a player that didn't toss the ball up. I don't mean he didn't toss it 6 inches, I mean, he didn't toss the ball at all, but rather threw it back towards his paddle. He was a penholder and was super pissed I even brought it up. He didn't stop, so I started blatantly hiding my serves! Hahahaa. He was better than me, but I won based on the serve hiding.
As for hidden serves in general, I wll say that I won't call it if I am winning easily, but I mention it to them after I win the game. If it's close, I'll call a let and let them know they're hiding.
My old practice partner was a guy who hid his serves, but legitimately had no idea. He worked on his pendulum and has now cleaned that up. It IS possible you might be hiding, but aren't aware of it, and that many of your opponents just don't mind.