well, you don't HAVE to buy 12x CDRs to be able to write them at 12x. you can buy whatever you like and write them at whatever speed you want. however, if you're writing at 12x, media that's been certified for writing at 12x probably won't fail as often as media that's been certified for a slower speed. i'm not saying that you won't be able to do it, just that when some (a lot? most? nearly all?) of your discs fail, don't be surprised.
and with media so cheap, why not buy the 12x?
-r.