I don't think it's necessary to refix negatives. Selenium doesn't rehalogenate silver to the best of my knowledge. I don't refix prints after toning in selenium so I don't see why negatives would be any different.
However, I do make sure that my negatives are absolutely fixed and washed before toning in Selenium. I think this is definitely more important.
Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.