Tipping is done at your own discretion. Theater ushers do not necessarily expect a tip, while waiters, tour guides, bartenders, and taxi drivers do. Rounding off bills to the next highest sum is customary for bills under €10. Above that sum you should add a little more.
Service charges are included in all restaurant checks (listed as Bedienung), as is tax (listed as MwSt). Nonetheless, it is customary to round up the bill to the nearest euro or to leave about 5%–10%. Give it to the waitstaff as you pay the bill; don't leave it on the table, as that's considered rude.