In my googling, it seems like people see anywhere from 0 to 6% with common modern FWD-offset wheels, and more with old school deep dish RWD-offset wheels. Guess what's on my Buick...
The front wheels from this truck are currently my Buick's rear wheels:
...and the original wheels are currently my front wheels:
They've got to be pretty wasteful, but they're also ugly. The original wheels wouldn't be ugly if they were in good condition but they're a bit beat-up, and the combination of two different wheels is bound to be ugly even if they're both nice (let alone mine).
Since Saturday I've been thinking more seriously about wheelcovers because I saw lots of wheels with like-new tires at the junkyard that would fit...for $25 I can get a wheel + tire and bolt it on myself and never have to deal with used tires (meeting someone from Craigslist, buying, then waiting and paying for mounting) or the cost (and shopping and waiting) of new tires.
The trouble with that plan is that they're potentially even more ugly than my current wheels, a shiny black steelie + a silver S10 steelie + a rusty Buick Series 57 on the same car might be so awful even I wouldn't be able to deal with it.