Why would employees in front of a counter be more deserving than employees behind a counter?
Or perhaps I should put it like: Why would a business need to pay a predictable market rate salary for employees behind the counter, but not for employees in front of the counter (because you step in and provide it instead)?
The the waiter/waitress had to come to my table, discuss the menu, provide feedback on questions, submitted order to kitchen, delivered order, checked back on us to see if we need anything else, etc. Level of interaction with staff > 1, level of effort from staff > 1. Tip appropriately if level of effort > 1 for helpfulness, politeness, attentiveness, etc. Stop making this so hard.
If anything it seems to me that it would be more logical to tip the person doing the more boring and repetitive work.
(My personal opinion is that tipping in general is a blight on a modern society and something that belong in primitive third-world economies. The way to fix this is via legislation: we simply make all forms of both receiving and giving tips illegal. It probably won't take much enforcement to quickly shift the social norms and economic practices once the wheels get rolling).