Depends on what you want to do with it;'navigation' covers a lot of ground). GPS work? Pencil and paper with a map scale?

I make maps (on a topo base) so when I'm out mapping, ground truthing, whatever, using a scale that's base 10 is much more convenient - especially if it's raining, windy, really buggy, or you're otherwise distracted. I've always used UTMs.

This also holds true for coordinate transforms, like if you're using legacy data that's based off a different datum. Software tends to be more agreeable using UTMs...or at least it's easier for me to see where I screwed up.

I understand that most state agencies (Alaska's one; I don't know about others) use decimal degrees, so if you want to plot a mine shaft or, more importantly, call for an evacuation, having decimal degrees expedites the process. SAR and LEOs please chip in with your experiences! This would be a very good thing to know.

And, as a GIS wizard told me a few years back, UTM stands for "Use This Method."