Mass sub: all the time, both offense and defense, usually in a blowout to start a drive if 1st in 4th qtr (i.e. I'll let my starters finish a 3rd qtr drive if it rolls into the 4th, but that'll be it then).
My reasons: initially, it was b/c I was convinced that playing time helped back-ups with progression in the off-season. I've never seen a definitive answer to that question (here in OS) so I partly still believe this. However, after TOO, too many injuries to starting HBs, QBs and DBs I'm always anxious to get those skilled starters out of the game. Played Bama in my Iron Bowl last night and had built up a 28 pt lead w/ a late 3rd qtr TD heading into 4th...Mass Sub! Don't want to walk into SEC championship game (and then, if win, BCS title game) having hurt a guy trying to keep a shutout w/ a 4-TD lead.
Some blowouts, played at San Jose State after the Bama win, I'll Mass earlier and roll down to 3rd string QB (true, Fr.; I do that often anyway, trying to split time b/t him and the 2nd string Sr. QB) and and 4th string HB (in both instances, holding on to my first thought that PT = even just 1 more progression pt in offseason training).