It was always my understanding, (and I may be wrong), that these files were there to prevent two users opening the same project from the same source.
I have never seen them any size other than 0 length, so am dubious as to their use as "swap" files, but then i've never monitored them while "working" with RSLogix5000.
Anyway, if RSLogix5000 is not running, and there is no possibility of another (networked) PC having the file open, then it is safe to delete them. They will get left behind if RSLogix5000 is terminated abnormally, or power is pulled on the PC. The next normal closure of a project will delete them anyway, so it's probably not even worth worrying about them.