Those "._" files represent resource forks, and are a hangover from pre-OSX days. They don't store anything of interest for music files, and they're certainly no use to Rockbox.
The Disktidy plug-in will remove them from your music player, but you'll have to run it after every file transfer. (And just plugging into the Mac will give you .DS_Store files in every directory you open in Mac OS.)
There's a shareware utility called
BlueHarvest , which prevents the OS from writing those files (and other Mac-specific hidden files) to non-HFS disks. (Or, more correctly, it deletes them after the OS has written them.)