The proper way to do things is to dither every time you lower the bit resolution. This would apply for exporting but also internally as well. Every time you use a plugin, or even move a fader, there can be rounding errors. The DAW is constantly going from higher bit depths to lower bit depths. You can truncate these errors which creates distortion, or dither and essentially make it broadband noise
Many coders are lazy and will simply truncate based on the argument that floating point will have the errors be so low they won't be audible. There is some truth to this, but errors add up, and eventually you will hear the "crunchiness" of excessive truncation.
I think everyone knows you should dither going to 16 bit, but many neglect the rest.