I don't normally worry too much about the paint job on the steel wheels. I think it's mostly the heat that wrecks the paint, particularly on the fronts where the braking heat is greatest. I do usually pretty-up the wheel that's going to be the spare, partly to keep dust etc. out of the car.
Give the wheel and tyre a good wash, then scrub the wheel with mineral turpentine to remove tar and any loose paint. Spray with suitable paint to match the other wheels; I use flat black and mask off the junction between the tyre and the wheel, and only paint the outside. Finish off with a coat of rust-proof, such as Tectyl, inside and out. The protective coating stops rust while the wheel is sitting in the boot and also helps keep the wheel trim in place, if it's a push-on job. By the way, silver plastic trims can be sanded and re-sprayed quite effectively if you need to get rid of kerb-rash.
If you want a really good job, it's necessary to remove the tyre, completely clean the wheel and prepare/paint as you would any other automotive surface. You would have to be keen to do this, as it requires your car to be off the road. You could do the job while getting new tyres, but it would not be very convenient.
It also depends on how much metal is showing through the wheel trim. If not much, then the wheels will be in the "Nobody will ever notice" category. One car I had for a while had small chrome hub-caps which left most of the wheel bare; the wheels never needed attention, so perhaps they were well painted from the factory.
