This is a tough call...…...I tried fishing one of the RI breachways last fall and everyone I spoke to was friendly and happy to let my pick my spot. I'll admit it wasn't crowded but these gentleman (young and old) were genuinely friendly and enjoyed chatting and sharing technique and tackle tips.
I've driven 3 hours to fish a specific pool and then moved over to make room for an 11 year old when I saw he was casting a sweet line. When he didn't hook up I gave him the right fly and I felt pretty good when the lad started hooking brookies.
I've also been crowded by people who didn't know or didn't care they were fishing too close. I just kept casting until they figured it out.
I always try to make fishing a positive experience for everyone if possible but I agree that 1st come 1st serve is the rule except for some places like salmon rivers where rotations are very traditional and sometimes quite rigid.
For comparison - I've seen similar issues regarding access below high water mark for duck hunting. I saw my cousin pick up a guides decoys that were set in front of his blind (on cousins deeded land.) My brother wasn't so polite...…...he asked once and then started blasting the decoys with his 3"mags. (if this seems extreme consider that the bottom there was oyster bed that he actually DOES pay taxes on!)
I would always like to recommend avoiding violence...…..but I may have a different answer if the bite is on and you cast across my line.
Hope this helps.....bottom line is it's situational in my opinion.