If you were to pack say 60 Lbs on one side and none on the other you might have some problems, but otherwise its no big deal. When I leave for work one side had cloths the other side has lunch. Lunch includes a thermos with milk, and possibly a few cans of pop (I take them in batches). I might have a 10Lbs difference at worst. I do notice it a bit when going over the speed bumps in my neighborhood so I take those slowly, otherwise you'll never notive.

Clearly your co-worker has never been to Holland. Just stand outside the train station in Amsterdam and I guarantee you that within a minute you'll see some dapper young lad come by and pickup or drop off a girl friend. The young ladies always sit on the super heavy duty european rear racks with their legs to one side... talking about balancing a large asymetric load. The Dutch has been doing this for over a hundered years. Clearly the bike is a pretty easy thing to ride.

Happy riding,
André