Yeah, it's mostly first come first serve. But it's also rude to grab a spot if someone is obviously getting ready to put something in it.
Here is an example of when first come first serve does not apply IMO: Once on a flight when I was sitting at the Y bulkhead exit (757), I took my carry on down after we reached cruising altitude to put on the floor during the flight. Unbeknownst to me, a pax across the aisle saw this and went back to the middle of the plane where he had been forced to put his carry on because everything was full in front near us. When I got up before landing to put my things away and realized what had happened, he rudely told me that I had given up that space and it was now his. Obnoxious, especially because I did not have any under seat storage room in the bulkhead. Thankfully, the FA saw what happened and offered to put my bag in the F closet for landing so I would not have to try to swim upstream to get my bag or wait for everyone else to deplane.