Tricky Dicky formulated the question :
Personally I do not double over in case sometime in the future a spur needs
to be added. I do tend to use known makes eg MK and have yet to experience
any problems of wires not being grabbed in sockets. The last time I had a
loose wire was in a wall switch which was a no name make my SiL bought.

So I would really like to know what the current professional opinion is on
the subject?

I was always taught as near as possible to fill the terminal, to
minimise resistance. If you double them over, then need to add extra
wires, you can always cut off at the fold.