Back in the day when I played D&D and AD&D, we rolled for monster reactions exactly never. Well, maybe someone may have rolled one once, while reading the book or something. In play, never.
Which is odd, because in one of the books we used there was a really remarkable rule about reactions.
Yet looking at the Tom Moldvay Basic Set, it's a really critical roll. You should be rolling reactions, at least by a strict reading of the rules, for every encounter with monsters. Doesn't seem to matter which kinds - slimes, dragons, crab spiders, hobgoblins who don't know who Gary is, etc.
Using the chart from p. B24:
Dice Roll / Reaction
2 Immediate Attack
3-5 Hostile, Possible Attack
6-8 Uncertain, monster confused
9-11 No attack, monster leaves or considers offer
12 Enthusiastic friendship
It's a flexible table, since you also use it for determining intensity of pursuit, and to allow re-rolls if the PCs try to establish better relations (you can see an example of that on p. B28). Charisma 13+ gives you a +1 to the roll; Charisma 18 gives a +2. A +1 means nothing is going to just attack you immediately, and a +2 skews your results to where a 10-12 is Enthusiastic friendship and a 7+ is at least "No attack." Nice. Especially in a game where treasure provides so much of the XP, the ability to basically waive off monsters with some nice talk is pretty potent.
(It's also worth considering with wandering monsters - you should be table to talk or bribe or distract your way out of fights about half the time, if you assume 6-8 is going to just force another roll.)
Where is GURPS DF in this?
My idle thoughts are, what if I rolled reactions for monsters every encounter in GURPS? The table split isn't terribly different, ranging from "Disastrous" on a 0 or less to "Excellent" on a 19+. GURPS has more native modifiers to reactions, too, which probably makes sense.
The modifiers on p. B561 range from a +1 to +5 for PC strength, -1 to -5 for NPC strength, -2 for being on the NPC's turf, and -2 for no shared language. Plus the PC's reaction bonuses and penalties, as well - and the NPC's, too. If orcs have Intolerance (Elves) you'll get their -3 on that roll. Foes with Overconfidence might knock your +4 for strength down to +2, or ones with Bad Temper might react well but be ready for any "insult" to set them off.
Appropriate actions would help, too - weapons away and friends words would be a +1 to +2, or use of a social skill roll. Of course it might encourage an aggressive creature who isn't smart enough to recognize "weapons away" as s sign of friendliness not weakness. Weapons out might put an otherwise friendly person on guard - if only because GMs have NPCs react like PCs, who regard any weapons on display as prima facie proof the owner is a violent psychopath who can't be trusted and must be killed.
I'd probably zero out any relative strength bonuses and penalties for unintelligent creatures.
I haven't dug around in Social Engineering yet, but I will. There might be useful material in there. I'm just thinking out loud this morning and I haven't had time to really read up.
It might be fun to roll out any reaction like this, just purely mechanically, and let the results stands and see if you can explain them. That slime isn't hungry, thanks to that 18 you just rolled that drops to 14 for "no common language" and "intruders." It just wanders off somewhere else, and you'll never know exactly why. The orcs might be neutral (say, on an 11) except you look weak compared to them (for a -2) and you blundered into "their" section and try to pass off Broken Goblinese as a way to speak to Orcs, for a total of -6. That's a 5, which is "Bad" and "attack unless outnumbered." Begin combat!
None of this is really "new" for GURPS, it's just me thinking . . . what if I followed the rules-literal reading of D&D Basic Rules and applied that to critters encountered in GURPS? In a way, GURPS is natively a lot closer to OD&D, which has a similar table but it seems to be for NPCs being hired, or monsters being negotiated with and induced into service.
Either way . . .
I'll try this a bit more. I do tend to just decide how the monsters would react and do that. I roll a lot for allied NPCs, of course, because Loyalty is a thing. Which is fine, but it's kind of fun to roll for monsters. Of course, I suspect most of the PCs have stacked up massive reaction penalties because, hey, who cares if the monsters hate you . . .