IMO it depends very much on the service rendered. If you serve yourself, there's really no one to tip and you might be expected to remove your own dishes or not see any staff members doing this. If someone makes an alcoholic drink for you, regardless of whether you pay or not, I would tip the same as in a bar (or airport lounge).

However, the lounges with the best service tend to be located in places such as Japan where there's a culture of no tipping.

It can be useful to observe what guests from the same country as the hotel's location are doing. If possible, ask friends and colleagues whose judgment you trust what the standard would be in their region in similar situations. Don't believe all the brochures for tourists that tell you how much to tip--I suspect they're written by people hoping to collect big tips.