Modified GEO method here.
The idea of the softener making the sides of the tank slippery made a lot of sense to me. I treat the tank with softener after a good cleaning a couple of times per year.
The rest of the time it is dump and rinse, rinse and rinse. The only time we have had a smell problem is when we were stuck at home for a while with no place to dump with waste in the tank. After a good cleaning and softener it was back to normal (no smell).