Preface: I'm a total tech head.

How often do i have to deviate? sometimes multiple times per day. You certainly wouldn't want to be stuck on your mapped route just because you don't have other resources for finding your way. things to cause deviations would be: road construction, poor original choice of road, seeing something interesting you want to go check out, finding a restaurant/store/lodging off your route, making new friends and wanting to ride with them, weather, illness, other random changes of plans.

If all you have to navigate with is a line on a blank screen, you will be really constrained.

You might find a GPS like the Garmin 800, 810 or 1000 useful, because you get an actual map to use (if you buy it, which you should) - but the 500 with a breadrcrumb trail will make you tear your hair out. It would be better to have a paper map.

I used a Garmin 705 on tour once (predecessor to the 800), and it was somewhat useful for navigating in cities where there were lots of turns. Once you are on the open road, it's not particularly useful. And in town, a breadcrumb without street name information would be worse than useless.

In cities, i find my iPhone easier to use for navigation than the 810, but the 810 is better than the 705 was.