1) not really necessary. And if SWIY spice smokes well, that's excellent. There is  no need to fix something if it's not broken, right?

2) the recrystallisation removes the dmt oxide as well as lipids. ( the stuff that can colour sthe spice yellowish. And washing will remove traces of the base that might have made it into the naphtha. It may also remove some dmt-n-oxide by virtue of it being slightly nore water soluble than dmt.