Given a 64-bit float DAW environment,
Bitter (FREE) is a useful plug-in which shows what a plug-in returns.
e.g. 1kHz sine generated in 64-bit float:
View attachment 49779
Following an EQ plug-in (filters all off):
View attachment 49780
Following a DIFFERENT EQ plug-in (filters all off):
View attachment 49781
Both are truncating to 32-bit float.
Most modern plug-ins are 64-bit float internally, but many will return 32-bit float back to the host application. Some are still 32-bit float internally.
Here is the older of the two EQ plug-ins referred to above, with a bell filter added:
View attachment 49789
And the more modern of the two EQ plug-ins referred to above with approx. the same bell filter added:
View attachment 49790
Some plug-ins that are 64-bit internally but return 32-bit float will dither before truncation. Those that return fixed point 16-bit/24-bit etc. will usually have the option to add dither first (sometimes noise shaping also, e.g. in the case of various Waves limiters.) This can be a problem, actually, because then you have to manage the subsequent gain structure around the added dither, i.e. don't start adding too much gain or the noise floor will be far higher than it need be!
Strictly, dither is needed for all operations but then we get into the question of dither for floating-point and certainly at 64-bit float, truncation effects aren't generally anything to ever be concerned about.
(Hopefully the developer hasn't got into a mess e.g. with denormals.)
Incidentally, when Cakewalk (SONAR) moved over to 64-bit floating-point, I think it was the first DAW to do so (or certainly one of the first) and someone from Cakewalk wrote an AES paper showing that 64-bit float was needed in a mixer to achieve strict 24-bit performance. (i.e. 32-bit float is insufficient.) The (in)audibility of the difference (for mixing, i.e. scaling/adding) is a bit irrelevant to my mind given 64-bit float mixing is
no problemo for a modern PC.
Yes, when exporting to a lower bit-depth add dither at the end of the "stereo bus" and not on every channel/sub mix bus--unless exporting "stems" (i.e. sub-mixes) or individual tracks to lower bit-depth.