I suggest that when a post is migrated from one site to another, that the question vote score get reset to zero (or perhaps only a negative vote score is reset to zero). Often people will downvote a question when the only thing that's "wrong" with it is that it's posted on the incorrect site. After migration, those downvotes hang around as a negative score and may unfairly bias the treatment of the question.
As Jon has suggested, a migrated question with a negative score will now be reset to 0.
I think just resetting negatives to 0 would be better than wiping it out completely.
If people have upvoted a question despite it being in the wrong place, that's an indication that it's a really good question - or possibly just an old question which had nowhere better to go at the time of posting (i.e. before Meta/SU/SF came about).
I don't see any reason to remove the community's approval of such posts - being moved to the right place is unlikely to make anyone approve of it less, but wiping out negative scores is reasonable as it may well make people disapprove of it less.
The only downside to this is the possibility that a user could vote on it twice, once on each site - I don't know if the vote "owners" are transferred via account associations.