I think it depends a lot on your geometry. On my touring bike, the more upright geometry means that it handles decently with some weight on the front. Touring with my road bike, I don't put any more than 6 pounds up front (pretty much a handlebar bag with the lock, tools, and small dense items) since the geometry balances my body over both the seat and handlebars. Maybe the way to look at it is to see how much weight total, of both your body's position and of the luggage.