I tip exclusively with $2 bills because bartenders/event staff/doormen/shuttle people then always remember me. I get them from the bank and they are invariably crisp and never been folded. They kind of stick together because of this, too.
Also, I've noticed that requesting $2 bills is an interesting test of your bank's ability to perform its job as a member bank of the Federal Reserve. I've had so many tellers tell me that $2 bills no longer exist, only to have their managers offer to call me when they arrive in about a week's time.