To me, the important thing is coming up with a good or interesting “Why?” that drives more play.
First, you have to consider any pre-established motivation: if the zombies in this dungeon are guards set by the necromancer, then a random encounter with zombies probably means they’ll attack.
Second, I think that Charisma modifiers and other modifiers should only apply if the PCs are actively engaging the creatures in the appropriate fashion. If the PCs are just standing there (or if the creatures wouldn’t give a whit about your Charisma; e.g. zombies, wolves, lions, black puddings, etc.), I’d make an unmodified roll. This is very important, because it weighs the results heavily to Neutral (6-8), where attack is still
Third, I’d roll and interpret the result. “Indifferent” for zombies could, indeed, mean that they just stand there, staring off; or, maybe more interesting, they’re performing some kind of odd, possibly pointless function: cleaning the room with brooms, performing military drill, waiting a table that isn’t even there with trays that don’t exist… meanwhile, the black pudding might just not register the PCs as food.
Even “Neutral” just means, by the book, that if the PCs parley, the monster MIGHT let them go (they’d have to be successfully bribed, intimited, or bluffed). To me, better results (12+, Friendly) mean they are willing to leave the PCs alone for no return, and are actually open to being hired or bribed to help, NOT that they jump to help them.
I’d also consider a multiple-roll system. The PCs bump into a gang of bugbears, and their first action is to close ranks and ready their weapons. I’d roll an unmodified 2d6 Reaction Roll. I get a Neutral result, and neither side is attacking the other yet… so the PCs decide to parley, using the leader’s (who has to be in front, or get in front - can’t very well negotiate from the rear ranks!) Charisma and Diplomacy modifiers. They get a Friendly result. The bugbears are now willing to let them pass unmolested (within reason; they won’t have free run of the bugbears’ camp, and if the bugbears find them later loaded with treasure, a new roll will be necessary… with modifiers if they look weakened!). Being Chaotic, the speaker decides to try to hire the bugbears, and makes another Reaction Roll…