However, if you're highly skilled with an impaling weapon, but not very strong, like Zaber in DF Whiterock, backstabbing isn't ideal, because most opponents have armor on their back. You'd really rather stab them in the eyes, which often gets you DR 0 and a free brain hit (for quadruple damage and a good chance at a knockout). However, if you can reach their eyes then they can see you coming, so they get a defense. (Quite likely at -2 for a side attack, but any defense is way better than no defense.)
Unless you're invisible. If you're invisible, and they don't have See Invisible, and they don't hear you coming or smell you, then frontstabbing should work about as well as backstabbing.
So my house rule on frontstabbing was basically: if you're invisible and approach an enemy from the front, it's a contest of your Stealth vs. their Hearing. If you win, they didn't hear you coming, and they will have no defense against your invisible attack. In DFRPG, Invisibility goes away after one attack, so only the first attack will get this benefit, but one good eye shot is often enough.
The other possibility is smell. For simplicity I don't give most opponents smell rolls, unless they have an exceptional sense of smell (like many canines), or the frontstabber is particularly stinky. (See, a reason for PCs not to dig through latrines for treasure!) The No-Smell spell comes in handy if you want to sneak up on scent hounds; Zaber actually used it to pickpocket a sleeping ettin guarded by a dire warg of his magic ring.