I would always go for post-blend myself. I get away with that because normally my household gets through enough coffee to justify it.
If you're going to pre-blend, I would say that you need to have roasted the single origin separately to understand how it roasts anyway. Pre-blends can work quite well (e.g. Malabar Gold, Espresso Vivace Dolce, Carolyn's blend) but I think the most fun as a coffee nut is to be had blending a number of roasted single origins in different combinations and proportions to see what you like best.
Just my two euro-cent's worth!