For some reason, I'm against having facing. It just seems to me that in 6 seconds someone can glance over their shoulder, and if they're hyped up on adrenaline they're going to be noticing and awful lot of their surroundings. It's not like a computer game where you march around with your nose always facing your toes.
That said, if you really want to use facing I'd advise using something like 5' of movement turns you 45 degrees, but only when you're running - it's otherwise a free action. Thus, you are facing in one direction when it's not your turn, which could make for some interesting scenarios with someone dashing across a corridor behind a raging melee with nobody noticing them. I'd still only give a stiff (-10?) penalty to Spot checks on targets out of your normal range of vision, because you're still moving around in combat.
But that's just me.