Punctuality is essential in Germany, and you may think that arriving on time will help you make a great first impression on your professor. But if you do come on the hour, you may be the only person there.
The “akademische Viertelstunde” is a longstanding tradition in German higher education. It originated in the early era of German universities when students would attend classes at their professors' homes. Because it would take some time to traverse the city between classes, students were allowed a 15 minute buffer.
Don't assume that you'll always have the extra 15 minutes to get to class, though. In many seminars, you'll be expected to arrive on the hour, while most lecturers give students the extra time to find their way to class. When you're signing up for classes, look for two acronyms.
"s.t." or "sine tempore" indicates that the class starts at the time listed (in German - ohne Zeit)
"c.t." or 'cum tempore" means that you have an extra 15 minutes to arrive for class (in German - mit Zeit)