No, I reuse the old seal and never had a problem. Only once, was the rubber a bit dry and damaged, in which case I replaced it. The seal sticks on the glass itself and should remain there normally. If it has become undone, use a bit of adhesive spray on the rubber to stick it back on the glass. But even if you don't do so and just put everything back together, the pressure from the clips will give a good seal. If you manage to find a Phase 1 spare headlight, most probably the adjuster will be on its way out anyway, you would be better off getting the repair kit and following DWM's excellent guide here:
https://www.volvoforums.org.uk/showthread.php?t=104675