The tip thing is quite interesting to me as well. It seems somewhat of a grey/gray area where there can be significant cultural differences as well.

What if some attendees want the tip to be atleast 20% while others think that 10% is more than enough? And isn't it an issue for the organiser to then have to force-choose one of the two options and then, perhaps incorrectly, get perceived as either too stingy or over-generous?