I often carry compasses and GPS units when I'm on a tour, but I never use them while cycling. They are strictly part of my backpacking gear, for use off the bike. I see no reason to ever use a GPS or a compass while bicycle touring. A decent set of regular road maps and a reasonable knowledge of how to use them should be totally sufficient. If you want to carry additional instrumentation it is totally up to you, but you should never get yourself into a situation where you are relying on a GPS unit alone.