i rarely weld. it either doesn't weld enough vertices, or it welds too many. instead i use vertex-stitching. you never end up welding too many vertices, although sometimes, the damn thing will decide welding a certain pair of vertices is not a good idea.
Yeah, the default Sew / Stitch in Maya is rubbish if you've ever used the Max one. However there is some smart stuff in Maya's UVs that I wish Max did - in Maya, it will only "weld" UVs if they share a vertex, so you can never do stupid stuff like in Max where you can weld two totally unrelated UVs and end up with a UV…