I never tip because that only serves to lower the wages of employees.
EDIT: The downvote button to this comment is actually an "I am a complete ass who thinks that living wages are something that I, the ever benevolent patron, should bestow upon the worthy serfs at my discretion"-button.
Thanks for the tip (pun intended), but I usually do not. However, most establishments that rely on tips are not upfront about it in any capacity.