I highly suggest using the carrying strap while on tour.
1) The bags close more securely when you do it properly like in the photo above, especially when less than full.
2) You can use the carrying strap for.... carrying the bags! Especially great for food shopping and airports.
3) You can remove the carrying straps and use them as compression straps for everything from strapping extra gear to your bike to tying your bike down on the roof of a chicken bus to keeping your pants around your waist after you lose fifteen pounds
Ride on!