Spraying varnish before decalling isn't strictly necessary, depending on how smooth your paint finish is, but definitely varnish over decals before weathering.
Decals don't go to well on a matt surface, so most people normally apply some sort of gloss coat. It doesn't have to be mirror like finish, just something not matt. Then decals, followed by another gloss coat, then the wash if you are using one. If you don't gloss coat over your decals, the wash can highlight the edges of the decals, you don't want that. The finish below worked well, as you can see, it's actually far from a gloss finish.