An ATP I know who flies a Varieze and a Defiant just indicated to me that this is common in the big iron - here's what he said:
"Didn't think this needed to clutter up the general forum, but one of the biggest uses of the offset function these days is for 'SLOP' - no joke. SLOP = Strategic Lateral Offset Procedures. The constantly reduced clearances between and along tracks in oceanic crossings (in order to stuff more airplanes onto them) coupled with increasingly accurate nav systems has prompted the procedure to be adopted by some operators where crews are instructed to fly a mile or two offset from the published track, as in NAT Tracks for example. The offset is taken out when reaching the waypoint where the flight coasts in with the first land based FIR / center. The "Big Sky" over water isn't nearly as big as it used to be.
The FAA and all of the comparable agencies owning the various FIRs are fine with it. Go figure."
Personally, I've been asked to offset to one side of an airway by ATC once, IIRC. Not common at all, IME.