There is only 1 rule. 1st come 1st served to capacity.
Anything else just shows how much you/others are willing to cater to others and is only a gentlemanly reflection.
That said, I have my one personal rule:
If I do end up staying at a shelter and it's to capacity, if someone who has come from a significantly longer distance than myself arrives, as the youngster with his own shelter... I'll cede my position to them. Has happened 50% of the time I decided to stay at a shelter.