There is no "rules" against any items.
It is absolutely good mannered to not grab other peoples wells (as explained by Alex in the first couple posts)
The user that throws an item in a well, should get the drops (as they are throwing an item the own into the well)
After that, no, there are not any rules against grabbing anything.
There are alot of slashers who will try and dictate that X is theirs, or Y is theirs, and there are even users who will kick you/ban you from runs if they do not get their way.
But, if the game was meant to have drops specifically for one user, then there would be a system in place to allocate drops.
there however is not, and drops remain free for anyone to pick up.