Just to been noticed, if you convert, you might get worse sound quality because they got compressed twice (just like if you edit a compressed jpg file).
If you have older iTunes songs using mp4, I recommered to use 256kbit or 196kbit (they orignally in 128kbit), so make sure it dont take even more info out of the song (you dont get cleaner sound quality, but its more make sure you dont get worse quality).
Best is updated to drm free versions of the songs.
So I my self never do that (except wma to mp4, so iTunes/iPhone can play them, but I do kept both versions).