Should I solder stranded wires when connecting them to a plug?
One should never apply lead alloy solder to stranded wires (often called "tinning") that are to be tightened under a screw. Eventually, the solder will cold-flow under the pressure and the screw connection will become loose.

This doesn't happen overnight; it can take months or years. I have personally seen this happen on several occasions.

It can be okay to just tin the very tip, to help keep the twisted wire strands together while tightening the screw. However, it is preferable just to leave some extra wire sticking out, and cut that off flush after tightening the screw.