All minis regardless of material should be cleaned before priming. The reasoning is because of the residual stuff left over on your mini from casting. If this residual stuff isnt removed, the paint on your mini will lift off very easily. It doesnt seem to effect plastics as much, but I clean all my minis before priming and havent had a problem since i started.
To clean your mini, soak in cool or warm (never hot, your mini will become very soft) soapy water. I use dish soap.
Let the mini soak for about 5 minis, then gently scrub it with a toothbrush. I would use an old discarded toothbrush. If you really want to, you could use your current toothbrush, but that is pretty gross
Be very gentle when scrubbing finecast. It is very brittle.
Once you finish cleaning the mini set it on a paper towel and allow it to air dry. You can expidite the drying process by gently pat drying the mini, but for the most part, it will need to air dry.