The real problem in answering this question is that video quality is a rather squishy concept. Certainly, a higher bitrate means less compression, and therefore less loss of data. But just what is "reasonable" is entirely dependent on the viewer (and in many cases on the video as well).

Myself, I'm an anime addict. A lot of my favorite shows are pulled directly out of the world of manga (Japanese comic books), and are therefore often drawn in a flat, 2D style, with many fairly static scenes. Thus, you can sometimes get away with a pretty horrible bitrate and still have a watchable show, because the video data can handle a great deal of compression.

In short, I'm afraid the real answer is that you'll have to experiment to find out what is best for you. Yes, the video quality will always degrade as you reduce the bitrate, but a certain amount of degradation may be acceptable (or even imperceptible) -- and that judgement is ultimately yours to make.