I have a tendency to fuse whenever possible, no matter what the material. It saves the possibility of a visible seam, as well as the dreaded finding of a seam if future sizing is anticipated. Though it’s how I do most joints, it’s just not an automatic go-to (for me) though.
The variables of types of stones , location relative to the solder joint and the possibility of mixed metals (yellow and white combinations for instance) all have to be taken into account.
Like Jo, any fusing I do is with a torch, The piece is heat sinked if necessary. Some metals are particularly easy, 18k rose, platinum, nearly any yellow gold will fuse well, silver requires a very practiced touch, but all can usually be done as easily as solder.
I’ve been out of the trade / repair world for many years, however I still see quite a few sizings from previous client work. I will always prefer to start with a piece that has not been soldered at the back of the shank, hence my perspective on fusing.
The primary issue is having a matching color to work with. When sizing down, I save the cut out section and roll it to about 1 or 2 tenths of a millimeter, making sure there is just enough tension to hold the piece I’ve rolled under very slight pressure between the shanks. The piece will overlap the shanks by a millimeter or so all around. Then it’s a matter of bringing up the heat . You’ll see the thin sheet melt first telling you that you’re near the fusing point
Then, go in a bit closer with the torch and the metals will join.
This is a very simplistic description I realize, but here practice is everything.
The advantage is a seamless and very strong joint.