at WSOP it is forbidden, however, if it is the last hand before B&T, or maybe a break, then I will do it if I really like the players.

In our rooms, its not allowed in tourneys, however, for some reason we have a policy to allow it in cash if the winning player of the hand says its ok....I know- strange - but it can not be abused.

I know it's not allowed.

I think it's rare that it reveals useful information about a players hand, maybe the nut flush card, or quad card appears and you can tell a player shoved with less than the nuts.

More often, a player finds out if they would have been results-oriented correct to make a call/fold. In some cases, they will tilt, other times they will play better.

When done, it's usually a subtle flash. Occasionally it's quickly shown face up on the board/upcards as they are gathered.

I don't recall ever asking in a casino, but I think I'd feel like an extra tip was warranted. Technically, the dealer could be risking discipline. I'll trust the dealers to know their own personal risk.