I think it is a matter of personal preference. I have tried just rear, just front and both front and rear, but if I am only adding one rack it will be on the rear. It just seems to work for me better but others prefer the weight on the front. Do you know anybody with a front only setup that might let you try it out?

My wife commutes on an old Fuji touring bike and she carries a ton of weight on the front and it doesn't bother her so it can work.