I wouldn't fret too much about the "cheating" part of it.  Even if you have it set to turn off under x mph the Garmin still shows your regular avg speed (not the "cheating" speed) - at least my 705 does.  In addition, you get both speeds on Garmin connect and it uses the lower avg speed when it runs reports and shows it in the listing of rides.  On my typical rides in traffic I usually get a .1 mph boost to my avg by setting it to turn off if under 5mph.  And besides slow vs. slow + .1mph isn't really gonna impress anyone.

Now if I forget to turn off the GPS and drive home that might really turn some heads!

I finally dropped my custom cut-off speed to 1mph.  I use this gps when I take the dogs out walking and at my former 3mph, it would cycle on/off all the time.