If you ever find yourself being given a comped drink of any sort whether it be the bartender buying you a drink or a promoter or bar manager giving you a drink ticket, consider yourself lucky. Free drinks are always an awesome thing. Keep in mind that free drinks are a gift, not a privilege. Obviously someone, somewhere on down the line thought you should be entitled to a free cocktail. Lucky you.
Comped drinks. Drink tickets. Free drinks. You may not have to pay for that round, but we (the bartenders) still have to make the drinks so please tip accordingly. In fact, since you aren't fronting the cash for the cocktail in the first place, show the bartender a little "extra" love and appreciation. Otherwise we are going to think you are bottom feeding pond scum, you will get served last and your drinks will only get weaker and weaker over the course of the night.
My rule of thumb and reasoning is if you can't afford to tip properly, you can't afford to go out so you might as well stay at home.