For Data CDs that you don't need to last 100 years don't bother...you CDRW won't be very picky about reading CDs on cheap media, but if you are making Audio CDs or something that will be going in other CD drive (specifically non-computer ones, and especially Car CD players) then you might want to consider high quality ones then.
As for speed rating you should probably get them speed rated to match your CDRW...but it doesn't always matter, like I have some old Kodac Ultima Gold CDs that are Rated at 4x because that was the fastest CDRW avaialbe when I got them, but they work fine at 8x in my new 8x8x24 drive.
However, I've also had CDs rated at 4x that give us blue screens on my roommates 6x...so it's kind of hit and miss...I'd buy CDs that are certified for my Speed...