Dealers don't really care since the money is pooled.
In 21, they should want you to bet for them for an obvious reason. If the player gets a double down or split, most put up another unit for the dealer.
So the dealer actually has the edge over the house in that a $5 tip bet made is worth MORE than $5 over the long run, since the player has to put up another $5 some of thetime.
When a blackjack dealer says she would rather have the sure $5, I immediately think "what an idiot". She is lowering the tip expectation, while simultaneously basically saying she thinks the player is going to lose.