In general you should tip a similar amount as you would in the U.S. Hotel staff seem particularly appreciative of a good tipper, and you are likely to get especially good service in return. If bills in restaurants include the word "propina" that is the tip, i.e., it has been included.  if you want to pay more, go ahead.
The tipping to get a good service is a wrong attitude, which twist the perception of the locals about where is the value of the service they should be delivering...There have been an article in the news paper Prensa Libre about this issue reminding the people that tipping is not mandatory, that the amount is related to perception of the service delivered and if you are presented with a bill including the "propina" you can reduced it (if you estimate the service was not up to your expectations) or increase it.