The main issue with rolling is musical:
1) play the bass note before the beat
2) play the bass note on the beat
If you play it before, this could mean the upper note will be played simultaneously with the melody. Thus, the resultant 4th interval (with the melody) doesn't sound very good in this context. If played on the beat, then there wouldn't be a 4th. I'm still undecided on this but whether or not it's on the beat or before changes depending on the measure because the intervals change. However, I've had satisfying results when neither of the bass notes interferes with the melody, i.e. the bass notes are played 1) before, and 2) the second note after the melody. It adds more musical texture this way.