As ghiribizzo has mentioned it's a matter of tailoring the profiles to suit your use.
One of my major frustrations with Basecamp was when I'd imported a route that I'd previously created in Mapsource I would want to have this in the motorcycle profile, well, I ride a bike so why not... Having just got a Zumo 660 that flips into the motorcycle profile as soon as you pop it into the bike cradle this became even more relevant.
Regrettably, in Basecamp, any of my previously carefully crafted Mapsource routes went totally hay-wire once the motorcycle profile was used in a recalculation. At this stage I cursed the Garmin development team that had tried to make Basecamp too clever and returned to the comfort blanket that is Mapsource. This all worked fine with my 'bikeised' Nuvi 765T and I was happy... a little frustrated, but happy...
Playing, and getting familiar, with my new 660 I loaded some routes for a forthcoming tour and ran one in simulation mode. I tried various things including a recalc. To my horror it changed the route out of all recognition. This was whilst it was in the motorcycle profile.
I started to look into this deeper and discovered that you could adjust the profiles, both in Basecamp and on the Zumo, independently of each other adding/removing different avoidances. The default settings for the automotive and motorcycle profiles, both on the GPS and Basecamp, were wildly different.
Any routes, created by other programs, imported into Basecamp have a profile called 'None' applied. The car profile is pretty close to this, the motorcycle profile has some significant differences.
For my use I'm happy with the standard automotive settings, it's what I'm used to. If I let the GPS decide the route for me and wanted it to avoid main roads (terminology is actually American so they say Major Highways, State Highways, Interstate etc.) then I can still set that if I want it. However, in the main, I use PC planned routes and set-up these up to follow the roads I want. I don't want the GPS interfering with this. However, unlike some, I prefer having auto recalculation on and expect my Garmin to only get me back onto my intended route and not totally change the rest of the route. To date my various Garmins have done this very satisfactorily.
Imported route, as it should be. Opening options menu to view profile settings
Automotive profile settings
Motorcycle profile (not sure on exact default settings but you get the idea)
Recalculating the route by changing the profile from None to Motorcycle
Route recalculated with the motorcycle profile (pretty messed up)
Having adjusted the bike profile to the same as the car profile and then recalculated
The settings on the Zumo are similar, you need to set each profile and then 'Tools' > 'Settings' > 'Navigation' > 'Avoidances'. You then check/un-check the boxes as required. Again running similar settings to Automotive should work. I just have un-paved roads checked on my bike profile, all others un-checked.
If you want to change these on the road then do so and then recalculate the route. i.e. if you wanted to avoid toll roads.
U turns iare always a tricky one as sharp hairpins are sometimes seen as U turns by Garmins, setting these up as an avoidance could play havoc with a decent biking route.