I'm not sure that IKM has a pronouncement on this, but I tend to aim to feed the
plugin -18 dBFS (not a terrible "rule of thumb" for many current plugins for many manufacturers), and then I adjust the input controls
within the plugin to hit the "virtual tape" etc. as hard or as soft as seems appropriate. Then I'll likewise tweak the output controls within the plugin to try to ensure that the signal coming out of the plugin is back at around -18 dbFS.
Basically, I'm trying to keep the signal hitting the plugins themselves pretty modest (i.e. around -18 dBFS), but of course,
within the plugin you can get all crazy with higher levels, if you see what I mean. (I think this makes sense, but ... if it doesn't, then hopefully someone will let us both know!
)