i found that it all depends on how you ask as to whether a tip is expected or not. if i walk up with my california accent and say...
"can i get a vodka redbull please...."
they look at you and wait for some more euros to float their way!
if i walk up and am a little vague with my accent, and maybe say....
"una cerveza por favor..."
a tip is not expected....but, being american, the feeling of guilt that immediately overcomes me when walking away without having tipped is something i would rather just have away with....so, i tip.
but, that said, by playing the tipping game....going back to the same bartender time and time again, tipping well....my drinks are ALWAYS stronger, always served faster, and i find myself being invited to more afterhours (if there is such a thing on that island!) than those around me.
always tip if you want good service. how much is up to you.