Not a dumb question at all! Just hemp/bamboo inserts unless you know they have prewashed them, some brands (eg DesignerBums) they prewash them so they're already pretty absorbent. Not sure whether the Wigglebums ones are prewashed or not.
(For making hemp/mf inserts, I found I had to prewash the hemp a few times before overlocking them together or it all got misshapen, so the ones I'm putting together for you will be good to go as soon as you get them

)
AFAIK cotton or microfibre inserts are absorbent straight away and don't need prewashing.