In the extremely rare cases of the dealer really peeking and not just looking thru a box, a tip is a great way to detect a tell. If you are sitting on 16 and the dealer has a 5 to go with his up 10, he'll let you know not to hit.Conversly, if he's got a 10 in the hole, he'll also let you know.
I know of only one dinky casino ,in Searchlight, where the dealer really is checking his hole card. Everywhere else, they are looking for a symbol and have no idea what the card is, only what its not.