I never use sims, I've tried but they get on my nerves too much.
I used to use slick tires a lot in the beginning if I could afford them, but lately, I've been using normal tires a lot for B-license and free races, just to help make sure I don't have too much of an advantage over the competition. I'll either use sports or street suspension parts, too. This goes for the Trail Mountain Enduro, too.
I'll use sport tires with sports or semi-racing suspension for A-license and some other more advanced races like I-C or I-B events, the Seattle Enduro, the Apricot Hill enduro, etc.
I'll only use slicks rarely nowadays...pretty much only for heavy cars or for full-race series like the World Cup, GT300 and 500, Tuned Turbo races, Rome Enduro, SSr5 enduro, etc.
None of this is set in stone, though. Mostly, I try and make sure I dont have an advantage over the AI or my GT2 obsessed roomate!