I think one of the strengths with Wavelab is that it’s extremely flexible in that you can do things in many different ways depending on what your workflow is. If you wanted to, for whatever reason, you could put dithering (or any other plugin) as a clip FX, a track FX, an output FX, in the master section… Compare it also with meta-data; you can put in meta-data for the project, but you can also specify it in the Format window, which then brings up the dialogue for meta-data. Editing the “Encoding” section bring up the encoding settings. What I’m suggesting is to put one more field in this section which brings up settings for dithering. This would be an OPTION, you could choose to not put anything in, same as with meta-data.
The benefit would be what I already described: being able to render out to several different formats (of which some require dithering) in one go without pausing to change settings in between.