In Oregon, titles on strictly OHVs are optional if you own them outright. Obviously they are a good idea if anything ever happend and a theft occurred or ownership dispute came up. Also, they may be required by the insurance company if you have it insure or the finance company if you have a loan on it.
Both of ours were purchased outright, so I don't have either of them titled, yet, but I'll get around to it pretty soon here, just in case.
As they can not ever be made street legal in Oregon, there is no vehicle registration. There are 2 year permits required for riding on public land, however, which is de facto registration, obviously. But it's cheap, only like $10 for 2 years, and it does not require that the vehicle be titled.