Rock Band 2's music store works more-or-less correctly via a cIOS, although some other games (GHWT, etc) may corrupt the songs or not work at all. It doesn't really matter, since you will encounter DLC problems with pretty much any method of backup loading. Playing from the original disc is the only guaranteed way to get the full DLC experience.
If you have a PAL copy of Rock Band, you are well-advised to keep your Shop Channel region at its default. Changing regions messes with your purchases and shop channel account. Yes, it doesn't have the full song selection of NTSC's store yet, but more than 50% of the songs are available and they update it with several hundred additional songs every month or so.