I left the ones on the inner sill open on Camprust, let it breathe, jacking outrigger ones give good access to cavity wax.
As far as seam welding shell....dunno? Is it gonna make a deal of difference on a road car?
Is it better to have a bit of flex on shell as whole, rather than possibly concentrating all the various forces onto an unknown weak link and cause cracking?......only musings, not my area of expertise , probably doesn't really matter either way, I'd say you've added a fair bit of lateral strength in the sill area with long butt welds.....take a plate of unwelded sheet and compare it to two pieces butt welded together, I know which one is stiffer, even ground back.