There was a time in the early 1970s when the only banana plugs I could find in the Toronto area were the solderable type (I still have some speaker cables with those plugs attached.)
It certainly isn't necessary, given the significant voltage/amperage of high-level audio signals from a power amplifier. If you were talking about the tiny output from a moving-magnet or moving-coil phono cartridge (a few millivolts--thousandths of a volt), then losses might be significant.
I haven't bothered soldering banana plugs for years. You could take this to ludicrous extremes, the way some tweaks are prone to do--and solder the entire signal path from the amplifier output terminals to the crossover board inside the speaker, but there's no significant--or audible--loss with high-level signals and typical speaker posts and banana plugs so don't worry about it.
- Alan
