As for fixed, throw the chain on roughly, spin cranks for a bit to knock off the excess wax and then do the tension properly.
I do this. Maybe I don’t do it enough. I still find the chain stretches a lot. I guess that’s wax on the interfacing parts wearing/compressing or something.
I've always waxed them. It takes seconds to run a fingernail around the groove to allow them to bite properly.
As for fixed, throw the chain on roughly, spin cranks for a bit to knock off the excess wax and then do the tension properly.