The ones within a short driving distance are worth the minimum because it's not even 10 minutes worth of work. I only did the ones in the rural area when they got to $18+.
I will do cards in 7/11 and gas stations provided it is a nice tight route. You can easily do 3 per hour, Kroger takes longer and needs more money. I like doing pre paid cards. Too bad the ones I do are quarterly.