I usually tip comped drinks.

Only time I don't tip full value when I use a coupon is at places where either (a) coupon use is so prevalent and standard that you're effectively overpaying by *not* showing up with a coupon or (b) the coupon had strings attached that invalidated something else, such as "happy hour".

An example of the former is a pizza joint where they've simply raised the price of the pizza to about 50% above "street value" because they've papered the neighborhood with so many coupons that nobody ever eats there without one. In that case, I just tip the bill normally.

An example of the latter is a place that is on Groupon often - but all of the non-Groupon diners there are getting 30-40% off anyway, and my Groupon is just getting another 10-20% above that. In that case, I try to guesstimate what I would have paid without the Groupon and tip on that amount.