I hate to say it, but unless I was planning on plowing it myself, or accepted the fact that there would be damage or extra cost to "not bury it" I would want it as far back as possible. I can easily see "a quick sweep/touch-up" being the one that snapped a post in the blink of an eye. I would think 8-10 would give a reasonable distance that most could avoid whacking it, but I've seen (and done) some strange things myself, so, no guarantees.
Are you thinking of running it the entire length of the drive/both sides? You may want to plan where the snow is going to be pushed to, too.
The idea of far enough "preventing damage from occurring" has a whole bunch of operators willing to test it.