Crush type washers deform to effect a seal. Additionally, the deformation may work harden the (metal) washer which along with the initial deformation, may affect the washer's ability to seal again. Most of what I own use aluminum or copper crush washers and I generally replace them each time, but I have smoothed and re-annealed a few in a pinch. At ~$1 a seal tops, cannot see a reason to not replace them.