The DMG has some really useful guidelines on creating monsters and assessing their challenge. But they seem to assume that all monster attacks will key off their attack stats or spells, and completely ignore the possibility of damage that does not require dice rolls (or that happens on a failed dice roll).
This can have considerable effect on the Challenge (CR) of a monster, and has tripped me up more than once. Are there any additional guidelines on how to calculate monster CR when it relies heavily on automatic damage or on effects with a save for half damage?
A good example is a comparison between a monster that has a normal attack that hits for 3d6 damage, versus a monster that is capable of casting magic missile. Both monster would have an expected damage output of 10.5 average per round, tagging them at Offensive CR 1. But the caster is vastly more dangerous because it will definitely deal that damage each round, whereas the other monster likely only has about a 50% chance of hurting anything.
Other examples include spells like fireball or lightning bolt or a dragon's breath weapon, but also auto-damage effects like a burning aura, an ooze's corroding touch, etc.
Best Answer
No guidelines exist in the DMG.
The DMG assumes that all attacks will hit, even if they have a chance of missing. DMG p. 278 ("Step 11. Damage", under the "Creating a Monster Stat Block" heading) gives the following example of a young white dragon:
(Note: I added up the numbers in the MM, and the multiattack adds up to 41, not 37...)
The only way the white dragon can average "37" damage per round with its multiattack is if it hits with all three attacks, which is not guaranteed. Similar language throughout this section indicates that you're supposed to assume that all the attacks hit. Therefore, it's impossible to make the distinction you're drawing.
Existing monsters suggest that CR increases.
The Marut in Mordenkainen's Tome of Foes (p. 213) is an interesting study for this question.
It has a listed CR of 25. Its defensive CR adds up to 28 (it has a 432 HP, plus an effective 90 from legendary resistance and x1.25 from immunities, a +1 to CR from having a higher AC, and a further effective +1 to CR from magic resistance), but its offensive CR is only 19 (120 damage per round from Unerring Slam). Therefore, its CR, according to the DMG calculation, is 23.5 (you can round it either way, technically).
However, its Unerring Slam ability is an automatic hit, and therefore it has an infinite attack bonus. Apparently this guaranteed hit is worth a CR boost of ~1.5, which puts it on par with some of the Monster Features on DMG p. 280. Based on this creature alone, you can probably estimate that guaranteed damage is worth a CR increase of 1-2.