i've learnt it's rude to leave your small coins like 1's, 2's, 5's!
no matter where you are, it's pretty not nice to give a waiter or a waitress this small stuff... either tip properly, or don't. the worst are the ones, being in a foreign country and not adapting to the tipping customs, even though it is a cultural element! For me, personally, if I'm in foreign country, I always tip between 5 and 10%. 10% is always safe, as it is American and so, pretty international. But watch out in Asian countrys: if you tip too much, it is like an offense for them...
And by the way, I've learnt in Spain, that it is not at all popular to pay seperately... Even though the waiterEss would earn more through second... 8)