Should cheap suspension forks be oiled often?
Lubricating the forks wont affect performance by may increase life and preserve cosmetic looks. Cheap forks typically have a coil spring and a sealed damper unit, neither will perform any better with lubrication.

Oiling the stanchions and dust seals (by Vorac's method above) will prevent dust and water from entering the lower, which over time would cause the fork to stop working, and will help keep the stanchions from rusting.

More expensive forks are different where oil dampers can get damaged if seals are not well maintained. The oil can become contaminated (different viscosity = different performance) and leak oil. Also any metal on metal could cause scratches which would allow to leak through.