JJB from Fairmont, WV on 6/1/2013 12:29:46 PM:
We had the exact same concerns as you. In all of our bike trips, not only on the C&O/GAP, we have never had a problem with someone trying to take our panniers or anything in them. We also carry money, ID, etc with us when we go into a business. I actually worry about my helmet and gloves as those are just hanging on the handlebars. We do try to park the bikes where we can see them. We always try to make it a little bit difficult for someone to try to steal our things. We only lock our bikes up at night. I guess if you are going to be in a building, (museum?), for a long period of time you may want to lock up your bikes.
I have to say that out on the trails and trail-towns, everyone has been very pleasant and "like-minded" and usually willing to help, (i.e., flat tire, asking for directions, etc.) I'm sure there is always a bad apple out there but we have been lucky and not come across him or her.