I usually do before tax... here at home, tax rate is about 8 or 8.5, so we take the tax amount, double it, and either pay that rounded up to an even amount, or add a bit more. That way, we KNOW the server will be getting at least 16% and usually a bit more.
If we sit for a looooooong time, they are extra great, or we have lots of special requests, we add extra always.
But I have been known to leave less than 15% if the service sucked. I feel no shame in this. This is rare, though. We usualyl have great luck with service... and have never had a problem with Disney service (knock on wood!)
For buffet, we do closer to 10% if all they do is refill drinks...