I'm not sure what you mean. The DM determines if there's a roll or not as the step after the player describes what they want to do. Everything follows from there.
Process 1
1. Player declares an action.
2. DM assesses whether the chances of success are auto failure, auto success or uncertain, and if using the guideline from the DMG, whether failure will have meaning.
3. DM determines auto success.
4. PC succeeds.
5. DM narrates outcome.
Process 2
1. Player declares an action.
2. DM assesses whether the chances of success are auto failure, auto success or uncertain, and if using the guideline from the DMG, whether failure will have meaning.
3. DM determines auto failure.
4. PC fails.
5. DM narrates outcome.
Process 3
1. Player declares an action.
2. DM assesses whether the chances of success are auto failure, auto success or uncertain, and if using the guideline from the DMG, whether failure will have meaning.
3. DM determines outcome uncertain(and if necessary that failure has meaning).
4. DM determines DC.
5. Player has the option to use things that might influence the roll(portent, guidance, etc.)
6. Player rolls.
7. DM determines from the roll whether success or failure happens.
8. DM narrates outcome.
Process 3 is very different from the first 2.
Certainly there are parts that are called out as optional including the section on table rules which includes a couple paragraphs on "metagame thinking." That a task needs a meaningful consequence for failure for there to be a check isn't presented that way.
Page 4 of the DMG.
"This book has two important companions:
the Player's Handbook, which contains the rules your players need to create characters and the rules you need to run the game, and the Monster Manual, which contains ready-touse monsters to populate your D&D world."
The PHB contains the rules to make characters and run the game. If you look throughout the DMG, the chapters are constantly saying, "The guidelines that follow..." or "The guidelines in this chapter..." or "The guidelines in chapters, 2, 4 and 5..."
The DMG is all guidelines. The rules are in the PHB.