I can’t tell you how many times while working as an event bartender in college someone would try and tip me. And while it would have been nice to accept the tip, I wasn’t allowed to. That’s because, at most open bars, the tip has already been taken care of by the person or group footing the bill.
While the inclination to tip is a good one, in this case, it can actually be seen as tacky. Your act of tipping assumes your host isn’t taking adequate care of the staff, which doesn’t reflect so well on how you view your host.
The one time you should tip at an open bar is when that bar is provided as an amenity, either at a hotel and resort as part of some all-inclusive package or inside an airport lounge. Here the drinks are being offered as a perk, or as part of an offering that you’ve booked. That package or perk does not include a tip for the staff, so tip here as you would if you had paid for the drink.