Assuming a single-aisle mainline a/c here….

A lot of the aisle seats have little room directly in front, so one has to use the middle under seat space if the storage room needed. The window seat has space directly in front for storage, but then one can’t really extend legs.

For myself, I assume in the aisle seat I get the middle storage and a very small footwell in front of me. In the window seat, I assume I get the storage under the seat in front of me and not much legroom - but I get the window and avoid getting knocked by passing bags if I board early.

At the end of the day, it’s a negotiation. Most of the time the arrangements work themselves out w/o even saying anything, but on rare occasions one might run into a difficult person.

putting feet on another’s bag is something I would not presume to do - or even ask to.