No rule is inviolate
Yeah, I apply these, including the DMG rule, and haven't house-ruled (yet). Firing at grappling targets might be the one thing I house rule.So does 5e, since creatures (including allied creatures) are sources of cover. Unless you have Sharpshooter, you need a good angle or your allies will grant your enemies half cover, which increases their effective AC against you by 2.
I just use the Hitting Cover optional rule from the DMG. If you miss a shot that would have hit if the target’s AC if they didn’t have cover, you hit the source of cover. Which basically means you’ve got a 10% chance of hitting your allies when shooting into melee without a clear angle.