Eh. That's bait, but I'll bite.
Because whatever metric or method is used to come to the agreement is irrelevant to the very question you asked, and the answer to your question is contained in the question. Rule 0 is "Have fun. Change the rules to have fun as you see appropriate." it is really that simple. Not everything is various shades of gray, some things are in fact black and white. What I think is fun and what you think is fun might be two different things, which may mean we aren't playing at the same table, but ultimately each table still abides rule 0 at every session. Seeking this difference without a distinction is splitting hairs at best, or if sealioning.
A wider, more beneficial discussion may fall to what methods of meeting rule 0 are "good" or "bad", which again is subjective. I don't like games where PC's can't die, I don't find it fun, but I also don't hide rolls as the DM on principle. I find it to be less fun when I'm just told "Oh, the ogre rolled a nat 20. Take 32 points of damage because he rolled max!" rather than seeing the rolls myself. Similarly, I use a home brew rule to expand on ritual casting which is as follows:
- Ritual Casting: All spellcasting classes that can cast spells as rituals may do so without consuming a spell slot as long as the spells are prepared. The casting time for a ritual spell increases by a factor of 2 (1 round becomes 1 minute, 1 minute becomes 10 minutes, etc) or as long as the ritual specifies. Under certain conditions, they may cast ritual spells from their spell list that are not prepared.
- Druids may cast ritual spells that they have not prepared while in unspoiled nature, such as a pond or grove, or under a clear and open sky.
- Clerics may cast ritual spells that they have not prepared when they are in a place of worship or hallowed ground related to their deity or domain.
- Wizards may cast ritual spells that they have not prepared when they read directly from their spellbook.
- Bards may cast ritual spells that they have not prepared when they have an audience of one or more creatures besides themselves.
- Sorcerers may cast ritual spells that they have not prepared by expending a sorcery point.
Some may not like this rule, because it "cheapens wizards being able to ritual cast spells they haven't prepared" or because I haven't figured out a way to have Warlocks do rituals, but my players like it and use it. Other rules I use are:
- Tumbling: The "Disengage" action is being replaced by the "Tumble" check. When you attempt to move through or leave a threatened space, you may roll an acrobatics check to avoid provoking an attack of opportunity. The base DC is 15 if you move at half speed (20 at full speed) increasing by 1 for each additional enemy that threatens you during the movement. If you succeed on a Tumble check, you do not provoke attacks of opportunity. Class features such as Rogue's Cunning Action retain the former Disengage action.
- Five Foot Step: As a movement action, you may choose to take a 5 foot step in combat. A five foot step does not provoke opportunity attacks, but it must be your only movement. A five foot step can be taken on the same round that you stand up from being prone in combat, but only after you have stood up, and is considered to be not moving for effects that require you to not have moved during a round, except channeling effects.
- Persistent Wounds: Characters who are downed in combat suffer lasting wounds which persist until a long rest. If a PC is downed in combat three times without a long rest, they succumb to these wounds and are killed out right. They may still be revived from death, but will revive with 5 stacks of exhaustion. At the completion of a long rest, all persistent wounds are healed.
Each of these rules my players have agreed to, because my players are invested in the world and the changes they provide to the game. I guarantee you that not everyone will like these rules, so they won't work for everyone, in particular players who don't want their characters to die or who dislike more mechanically intricate rules like tumbling. It is what it is, but ultimately that's a different conversation and question than what you've asked.