Logically, etiquette for the middle seat would be for the window pax to use it, as then they can access it without the aisle pax having to get up (in turn, the aisle pax can easily reach into the bins). But logical doesn’t always translate into real life.

I always board close to the end and rarely struggle for space. I check my main bag and only have a small carry-on, which will fit under the seat if it has to, or more often can be squeezed into a locker near me.