Point to ponder - exhaust companies put miles of weld beads on exhaust pipes every day, and never fill the tube before welding. Numerous alloys, often mixed materials in the same weld, they never backfill the tube or converter body or whatever is being welded. NOT filling the tube is standard practice in an industry that does this stuff every day and then puts a warranty on the finished parts. A decent weld that doesn't break is just as good as a perfect weld that doesn't break.
When I say "never", I mean I worked in the industry for 30 years and never saw a tube being backfilled on a production line. A lot of years ago I had a prototype system made out of titanum (rejected material from the nuclear industry) and the welder may have backfilled that, I've forgotten, but it is the only time in 30 years I remember this even being talked about. So ok, if you're welding together a titanium system for your Virago then maybe you will need to take precautions that aren't typical for an exhaust system. Let's assume you aren't using titanium in this discussion.
For the last couple years a big part of my job was breaking exhaust components. Welds were a major part of that. I tested thousands of welded joints to failure, none of them failed due to impurites being sucked in from the backside of the weld. If a welded joint failed too soon, the welds were dissected, polished, studied under a microscope. Impurities in the weld weren't the issue. The fix was never to start backfilling the pipe.
If your exhaust weld breaks, it won't be because you didn't backfill the tube. If you're sticking together two used pipes with a $150 harbor freight welder in your backyard, backfilling the tube isn't your big worry.
I won't argue "best practice" if you want to obtain the best possible weld. But the thing is, you don't need the best possible weld for an exhaust pipe. It's not being used in a nuclear reactor and the pipe isn't titanium. It's going onto a Honda CBwhatever and the pipe is mild steel or maybe some form of stainless.
Clean the material. Make the joints fit decently. Get your settings right. Take a class/get instruction from someone who truly knows how to weld, and practice first. You'll be fine. Backfilling is not a fix for ANY of the previously mentioned details.
Would you expect body guys to flood the back of the panel with shielding gas when they put in a patch panel? No one fills the frame tubes before welding them. Do people fill the swingarm before welding a brace onto it?