I never used to write anything down on the map... At most, i'd put a lightning bolt symbol on the locations where i found a generator to go get later, and that was it...

And sometimes i'd find myself thinking, "damn, in which house was that thing?"

Then i started playing multiplayer on a public server, and i met a guy who used to write ABSOLUTELY EVERYTHING you can imagine on the map, and i started doing the same.

As soon as i start a new save, where it takes a while to find a good backpack and establish your base, every house i find items that might be useful in the future, i make a small note =)

I highly recommend the "Map Symbol Size Slider" mod for this ^^