If it's not optional for rewarding exceptional service, then it should be part of the price. If everyone pays the same expected tip, there's no real benefit to having a separate line item on the bill.

But practically, certain customers end up paying a larger share of the service worker's take home pay. Dynamic pricing based on customer's willingness/ability to pay is an interesting pricing model, but in this case the risk is entirely on the service worker who hopes enough people pay sufficient tips.