You certainly don't have to, but they're cheap enough that you can, if you so choose. I did so largely in order to keep things straight when learning the difference between different blend types (codger burleys vs. Latakia-forward blends vs. Va/Pers, etc.), so that I didn't have to concern myself with whether a perceived difference was, in fact, just a ghost of a previous blend.
Now, it's not as big of an issue for me - but I still have my dedicated cobs, and will probably add a couple when I decide to really get into straight Virginias.