Hello there, here's a couple of related ideas that could be helpful:
-some mono plugins will 100% switch any in-coming audio to "mono", if originally the input source had some amount of left-right material simultaneously (stereo). This side effect could perhaps be of niffty utility if you desire to change a stereo track to mono with out much hassle (like DAW rendering with panning tricks to change source content etc)
-some mono plugins will not 100% switch in-coming audio to "mono", if originally the input source had some amount of left-right material simultaneously (stereo). This second possibility will occur if the plugin is designed to only process the mono content of the in coming signal, not necessarily eliminating simultaneous left-right present content. One example (i stress this is just one possibility bc plugin design varies so much, depends on coding intentions/errors etc)--- a mono compressor plugin that only "processes" any present mono content sent through it could perhaps only apply mono compression to stereo tracks such as a pair of summed piano microphones, here the mono processing compressor plugin could lack the ability to independently compress signal variations between the left and right (frequencies per panning/channel side). Here the compression applied would be no different than link left-right channel processing on a stereo compressor. There are actually too many variations of this type of panning-compatibility issues to go through all...
If I could suggest apply these ideas yourself, I would say your decision could relate to each individual track. A good plan=mono plugs on mono sources, stereo on stereo. Perhaps obvious ideas but stereo content present on one channel could benefit from a stereo processor plugin, or if the same "track" has individual left and right channels in the DAW then stereo plugins would not be 100% necessary as any panning on the mono tracks would (very likely) be post fx or inserts (*thus input into the mono plugs would still be mono, and panning post channel inserts would then change the mono to stereo). And again, for example, just watch your VU meters or whatever trackmeters you have and see if your stereo sources accidentally switch to mono with certain plugins...or just use the ol trusty ears here and there *wink wink.
wishing all the best
-MadHat