Originally Posted by Arcanum

Balance is generally good, but exactly how unbalanced you can get depends.

Different racks have different weight limits, though I doubt you're getting anywhere close to the rack's rated limit, much less the actual physical limit. A rough guideline is to take the rated limit of the rack and divide it in two. That would be a reasonable weight for each side. In reality, you can load more on one side than that without compromising the rack.

Balance in terms of the handling of the bike varies even more. Different rack/bike/bag combinations will mount the weight higher or lower, resulting in a higher or lower center of gravity. Higher center of gravity = less stable = more problems with unbalanced loads. Lower center of gravity = more stable = fewer problems with unbalanced loads. There's no easy rule; it just comes down to what feels comfortable with your set of gear.

A backpack, while inherently balanced, raises your center of gravity quite a bit, making it less than ideal for heavy loads. A more-or-less balanced arrangement in panniers will be more stable. Plus, panniers mean no sweaty back, and no bag possibly obstructing your view when you look over your shoulder.

What are you carrying (and how heavy is it) that makes this a problem?