In D&D 5e, spell save DCs can be any number based on the calculation (Player's Basic Rules pp. 22, 30):
Spell save DC = 8 + your proficiency bonus + your Wisdom modifier
Spell save DC = 8 + your proficiency bonus + your Intelligence modifier
However, the DM screen has a table for setting skill DCs for tasks, which are all divisible by 5 (I don't have my screen to reference the exact figures).
Are there any reasons not to set task DCs to anything between these defined, round numbers?
The reference table shows descriptors for DCs of 10, 15, 20, &c. This is just to provide you some touchstones--you can set intermediate DCs any time you'd like.
(However, the fine-tuning by 5% probabilities that provides isn't, in my experience, that useful and increases my cognitive load more than necessary. I've generally been happy with just using the "major" DCs.)
If you've considering setting a DC much lower than ten or upwards of thirty, please read 5 Simple Rules for Dating My Teenaged Skill System by AngryGM. It's an excellent expansion on the DMG's section "Using Ability Scores" (p.237) which says, in part:
Only call for a roll if there is a meaningful consequence for failure.
When deciding whether to use a roll, ask yourself two questions:
- Is a task so easy...?
- Is a task so inappropriate or impossible...?
(Warning: Angry's excellent advice and analyses are salted liberally with rude and vulgar language.)
DCs are whole numbers, the whole game plays in whole numbers - hence the round up or round down when dividing a number. The example numbers in the DMG (p. 238) are guidelines of how difficult a skill/save is.
5 - Very Easy
10 - Easy
15 - Moderate
20 - Hard
25 - Very Hard
30 - Nearly Impossible
It's up to the GM (even with a module, the GM can adjust) what the difficulty will be; it can be any number. Typically an NPC will have its own attributes to have their Spell DC, resistance to intimidation/persuasion, perception, stealth, etc. The DC would be calculated by their ability modifier, proficiency bonus, and possibly a base #, which will commonly not be in the set divisible by 5 (but it can happen).
Example: (Basic Rules, DM, p. 29)
Giant Wolf Spider
STR DEX CON INT WIS CHA
12 (+1) 16 (+3) 13 (+1) 3 (−4) 12 (+1) 4 (−3)
Skills Perception +3, Stealth +7
Senses blindsight 10 ft., darkvision 60 ft.
passive Perception 13
No
This serves as a general rule of thumb, to give you an idea of what constitutes an easy or difficult check, based on the overall balancing of the game. However, the DM can increase or decrease a check by any arbitrary amount.
For example, in the video series Critical Role the DM (Matt Mercer) often changes the DC of a given check based on the actions of players. For instance, a player delivering a genuinely convincing speech might have an easier time on a Deception check than one who mumbled something ill-conceived.
