cache file permissions

Cache files take whatever permission your uploads directory currently have. Your server should be able to write and delete them at will.

You can use 644 as long as both PHP and your web server run as the same user, and your files are also owned by the same exact user. Most host providers don’t do that, therefore by default, 755 is the logical choice, provided there is site isolation.

If each site is isolated from each other (not shared server) then this is irrelevant.