I'm with Ignacio:
It's not customary in Spain. You don't need to do it, and I believe it's not one custom we should import. I hate having to tip in some places only because the tourists' customs have made it popular, like taxis or bars. Fortunately, an inmense majority of spaniards don't tip at all anybody at the hotel.
But I stay mainly in 'mom & pop' type Hostals at which the Owners or the Owners' family do the chores.
As for tipping in restaurants: If there is a 'Service' charge on the bill, my top tip will be about 5%. I don't know if that 'Service' charge finds its way back to the wait staff but I think it should.
I don't tip the bartender if I have a drink at the bar but I might tip a waitress who serves me at a table.
I really deplore the fact that Big-tipping (mostly American) tourists have led many Spanish service-providers to expect tips for just doing a small service, thus ruining things for those of us who prefer to follow the lead of our Spanish hosts.