The general rule for tipping in Germany is that you round up to the nearest Euro unless the service was bad.

If your bill is 14,30€, you round up to 15€. If your bill is 14,80€, it's prefectly acceptable to round up to 15€, but I usually go with 16€ unless the service was bad.

In taxis, I round up to the nearest Euro so that we (both the cabbie and I) don't have to fiddle around with small change.

If I take advantage of a hotel porter service (which I rarely if ever do), I usually tip a Euro or two.

Other than that, tips aren't really expected. People are paid well enough that they are not dependent on tips - not even the waiters. Don't ever feel bad about not tipping somebody if the service was lousy. You're not taking food off anyone's table, and you've sent a signal to the person providing the service that they need to shape up.

That's why tipping still has its intended function in Germany (and most of Europe actually), which is to reward satisfactory service.