I have a fairly extensive roster of freight cars, some with metal wheels, but most with plastic. The only wheels that have given me a problem with gunk build-up are Kadee metal wheels. It's not causing enough of a problem to bother changing them, though, and the only time that I have to clean track is if I've been doing scenery work in the vicinity. That's in the 12 or 15 years since the layout was first started.
A too-common problem with plastic wheels is that some are out-of-round. I have a good supply of spares on hand, left over from doing wheel conversions for others, and can usually find a suitable replacement, but if not, I'll spring for metal wheels.
The truck tuner tool is a good investment: even old trucks can be rejuvenated with this handy gadget. As mentioned, a puff of graphite afterwards, into the journal area, will also help. The truck tuner only works when being turned in a clockwise direction, and you have to do each journal separately: that is, after doing one, remove the tool and turn it end-for-end, then do the journal on the opposite side of the truck from the first. An eight-wheel car requires eight separate operations with the truck tuner.
Wayne