From what I've heard it doesn't affect aerodynamics very much,and who cares about Fred.However fitting a handlebar bag on a bike that has brake lever shifters can be a hassle.The shifter cables come out the side and get in the way.There are adapters that you can use to reroute the cables so that you can mount a handlebar bag.
There are other options too,saddlebags such as Carradice bags can be mounted to almost any saddle(they do asy that you need saddlebag loops like those found on brooks saddles but that is not nessecary).They can be mounted to regular saddle rails if need be.
I would stay away from seat post mounted racks a bags though because they do effect handling,if thats even possible with your bike.