Wow.. That's suck...
My sister worked as Waitress and I'd been seeing how earn she gets money from paycheck and tips... Tips always being very high than paychecks because she earns tips everyday when she works but paycheck in every two weeks and it's still low and not enough for her bills, gas, kids and everything. Good thing that they do get tips to help them out than leaving them suffer...
For me to leaving tip, just more depend on how their works and respect for customers. I will decide from my mind to leaving how much tip for them if their work is great and their respect for me is good enough then I will leave more than 5 dollars or so, depends how much on the food that I pay for. If lousy then could be one dollar or nothing with the note to tell them the reason why I leave them a dollar or nothing, they will understand and improve themselves. I think it's helping.