Always. My parents drilled it into me from an early age--now it's second nature. Sit down, fasten belt, turn key. I don't even think about it anymore. Also, I find that in any abrupt manuver, lack of a seatbelt severly hampers your ability to accurately control the vehicle.
Aside from that, I spent four years with the rescue squad back when I was going to UT. I've seen more fatal MVAs (motor vehicle accidents) in a single day than many people will (hopefully) see in their entire life. Yeah, I've seen a couple drunks ejected, land in a field, and walk away with superficial injuries. I even saw a couple that arguably would have been killed had they been wearing their seatbelts (very rare, that).
But the number of people I've seen crushed, decapitated, or smashed into trees because they were unrestrained and ejected... I think I lost count after about the first dozen.
It ain't pretty, guys.
--Ty