We booked online with Virgin last time we stayed at ASM. After I booked, I phoned them up, requested the Dalmation block (due to my dalmation mad DD) and that's what we were allocated on arrival. Whether or not it was pure luck I don't know but i think Virgin do their best. From what I understand, a prefered room is one that is allocated close to the main Facilities or pool view etc. I don't think these are worth it at ASM as all the blocks are fairly close to the reception, food court and pools.
Nick