It can be good or bad depending on the situation.
If you are recording MP3 files from different sources that were created with widely varying basic volume levels, normalization can be useful in "leveling" the basic volume somewhat. This can be particularly useful in pop music, which typically doesn't have a wide dynamic range. Normalizing this type of music usually won't have unnatural sounding results as typical original recordings may use near-peak sound levels.
On the other hand, some music like classical music, ballads, or other musical passages that have a wide dynamic range can have the entire character and musicality of the passages ruined by careless normalization.
The bottom line is, it depends on the material. If you want to try this, I would suggest that you normalize the music and record it to hard drive first. Listen to it on the hard drive, and if it sounds OK to your ear, then go and and burn it. If the volume balance doesn't sound right, you can play with the normalization of individual tracks with most audio editors. Experiment with it, and try again until it sounds right.