Depends on the finish you want. If its a vintage patina'd look, or traditional factory look, then I would agree with sandblasting; however I wouldnt suggest coarse sand or glass beads. They will clean the aluminum and also "de-skin" it, giving a rough finish. Most valvecovers and intakes retain a smooth light gloss finish. Use walnut shells, TIP aka Truman's sells them.
I have my cabinet set up with walnut shells as I do a lot of Muncie 4-speeds and they are really dirty. The walnut shells remove all the crud and lets them look exactly as new.
If you use glass beads or sand, you might want to at least give it an oil rub or a light steel wool run to get some of the natural look back if you are doing a period look.
However, if you like the rougher look then glass beads work well.