Never. gun vise, lead sled (a closed system) introduces one more variable that you have to troubleshoot if your groups are wild or if they go wild when not using it.
When I'm going to do a lot of large caliber shooting(like 375 Ruger) from the bench, I use this--http://www.browning.com/products/catalog/clothing/detail.asp?fid=A024&cid=309&tid=013
They may last forever,IDK, I'm still using one that came with a vest 28 years ago.