I.D.,
I'm surprised that you didn't read the original question more accurately!
He's not asking about buffets where there is a guest check for dessert...he's talking about free breakfast buffets provided by some hotels for their guests. There is no guest check, ergo no service charge. 
IMHO... you should leave a few coins on the table for the bus and wait staff. Often, these employees actually work harder during buffets; the dining areas are usually full, and there are more tables to attend.
Again, this is a matter of personal choice. I completed my 2006 September trip with 3 nights at the Presidente (2 breakfasts, as I had to leave very early after my last night). I think I tipped the waitress 500 colones or so, but thanked her personally for good service. It's up to you.
At the risk of overstatement, I will say this: In general and for whatever reason, most gringos overtip. And I mean grotesquely in light of local customs. I really don't understand what guys are trying to accomplish by tipping 50% in an MP, tipping a BM/KL girl who is already charging top dollar, or guys who throw an additional 15-20% on top of restaurant checks as described by Irish Drifter above. It's just plain crazy!
So what's the issue for overtippers? Do they want to appear to be big shots? Do they feel sorry for the locals who live on the local economy? Or are they just plain profligate in their spending habits?
Regardless of reason, it is foolish to overtip in Costa Rica. All this practice will accomplish is to raise expectations on the part of service providers (note the recent behavior of some MP girls, demanding a big tip up front) and the often lousy service at many Gulch locations, particularly the Blue Marlin, where waitresses and bartenders receive immense tips from swaggering, drunken idiot gringos.
Yes, tip if you must, but please be aware that your tipping behavior affects other mongers and tourists.