It is safe to play NTSC on PAL, and PAL on NTSC. If you play the games from HDD, the process that WBFS Manager does with the ISO will strip it from any updates.
As it has been said before, it is the update (if you are on PAL, and update with NTSC-game, and vise-versa) that will brick your wii. Priiloader have an option that disables that function, and than you are safe.
As a rule, dont ever update your wii, unless you have read it on wiihacks.com. :P Thats my way of ceeping out of bricking my wii.