Consistency in placement of the forearm on the sandbag is what's important and the swivel shouldn't affect anything.
When I've tested my hunting rifles on the bench with sandbags, I like for the bag location under the forearm to be where I'll put my hand when shooting in the field. That's generally just behind the sling swivel--so that's where my front sandbag is always located. Consistency.
You're from BATFE? Come right in! I use all your fine products!