Every change should be treated the same : you tell about them at character creation and you tell them during the game while allowing for their set of rules on the present session if you cannot think of them in advance. Homebrew, legal rules, anything should be the same. It's not during a game that you tell the multiclass druid cleric that the steroid goodberries dont work in your game, as he's trying to heal someone after a fight. This actually happened to me. Don't fucking nerf the core of a character's mechanics midgame.

Yeah that's fair. For the most part we're sticking to 5e, and the consensus is always to check the rules first when we're unsure about something and to try and implement it as intended, so we're not losing any of what I'd consider to be core rules, like the way movement, actions and bonus actions work during combat, or spell slots and class-specific rules etc.

It's more of our approach to more niche elements, such as the food and water needs relevant to each creature's size as specified in the DM's Handbook - no one has the inclination to track our food supply and consumption to the pound per character, so we instead stock up on provisions to last X number of days, and track our usage by the day. It's just a bit quicker and easier to manage that way, and we can still implement the same effects in the event we run out of food.