You should never tin the wires! Solder is soft, so even when you tighten the screw after a period of time the soft solder will migrate and the screw will become loose.
Indeed. No doubt we have all come across many examples (I know I certainly have).
I believe that mains cables are still available without any plug fitted ... and, guess what, some (at least) have been seen with wires tinned by the manufacturer, whilst others may be bare.
Meanwhile, what do we think about the other (non-plug) end of a mains cable? Not all are the IEC C13 type (and may be "free" - with stripped bare ends, or even un-stripped - waiting for technicians to do their worst)!
Personally, I remain unconvinced that ferrules really achieve much ... and often need to be cut off when fitting a new plug if neatness (correct length of conductors) is to be obtained. And ... other considerations may apply when replacing non-British mains plugs.
On a historical note ... I can remember when tinning the wires was "encouraged" by trade instructors.
