Real time plugin processing is executed in the internal 32-bit floating point realm no matter the bit rate on your source files.
Again, what you say only applies when doing a lot of offline processing.
Dithering is only used as the last step (once), when reducing the bit rate to the intended format (ie 24 bit to 16 bit for CD).
Said that, some old plugins/plugin technology has dithering on by default. Which only try cover up bad (or once good, but old) plugin beaviour/programming.