I remember the first time I took a taxi-cab, the driver asked for £11.90, so I gave him £12 and then asked for my 10p change. The look he gave me was SO filthy that I realized right away that I'd done something horribly socially inadequate. I later asked my mom about it and she told me that it's pretty much expected that you should give the driver a 'tip'. I was so humiliated at the faux pas I made that since then, I have always made sure to give a tip to anyone who provides me a service, unless it clearly says 'service included'. I've been told I'm too generous with my tips... supposedly it's 10% - 15%, depending on the quality of service... but I usually give more because I'm afraid of getting a dirty look again. Better to be safe than sorry... right? 
_________________
~I wanna fly high, so I can reach the highest of all the heavens
Somebody will be waiting for me, so I've got to fly higher~