Something else to think about, in Europe the expectancy is very different to the US. Over here we tip for good service & not simply because it is expected.
I know that this can cause a lot of issues when European's travel over to the States & don't even think about tipping because we are not used to it.
Most staff over here are paid a proper wage (see the amount we pay for a meal!!) & the tip is seen as your way of saying thank you to the staff for a job well done.
In general we do not tip taxi drivers (again ask for a rough price before taking a taxi, but make sure you are sat down !!), and for tour guides & drivers it is quite the norm to simply put loose change or a couple of folded up notes into their tips box, or hand.
Don't worry, no-one will think you are being tight, and most people won't bother leaving anything
Have a great time,