So looking at the poll results, it seems that people mostly land on the "average, but if you need more hit points, just give it to them". Comparing this to the results of the other thread about player hit points, I can conclude that playing it by ear is the best. If I need more hit points, I can just increase them. And vice versa.
I need to see how the group handles encounters. If everything is a meat grinder, then less hit points. If feature villains die before they can do anything, maybe a bit more.
When I first started playing 5e, my eyes bugged out at how many Hit Dice monsters got sometimes. CR 5's with 15 HD! WTH!?
But now that I'm working on encounters more, it occurs to me that those 15 HD are only telling part of the story, since really, it depends on the size of the critter, and their Con can be just as much a factor if not more when it comes to hit points. It's a little strange, sure, to see a high Con critter walking around with less Hit Dice than a low Con critter- like Con should only be a factor for saves, not hit points!
And adjusting Con up or down can have drastic effects on enounters. No real amazing insights here, just an observation. Another odd thing was how the game handles enemies with vulnerabilities, by giving them more hit points! I don't know if that's universally true, but I've seen it on a few monsters now.
Which is really weird, because it assumes the party will have the ability to monopolize on said vulnerabilities. Also, I've seen lots of monsters, like Fiends, who have tons of resistances, and still have a big chunk of HP.
Even though the designers say they don't take accuracy and AC into account, AC is a factor in how tough enemies can be, but it can be less of one than you might expect (compare MM Ogres vs. Orogs, where the balance point isn't hit points as much as it is high AC vs. high damage).