What I do is I remove them from followers before midnight calculations roll around, on the paranoid chance that keeping them as followers when midnight hits means they are not treated as janitors.
Hmm, at least here you are not paranoid. As it is the normal jobs all use the dayblock variable and follower set the dayblock variable to 999. This means that for nearly any and all jobs (but not titles) you do need to remove the follower from its slot before the midnight calculations happen - with the sole exception being Nicole working in a newsroom (which she will do, even if she is following the mc/pc around all day and even at midnight).
Be aware though that Grimdark (GD-Studios) does in at least one case confuse dayblock 20 (Tavern whore) and dayblock 25 (Gladiatrix) - when the slot of a tavern whore (dayblock=20) is cleared (passage ClearSlot is called) then the variable $arenanpcs is increased by 1 (meaning that it is theoretically possible to have more than 6 female npcs signed up in the arena).
Please also note, that the usage of "giving a day off" and having a npc in bondage during midnight calculation is not handled consistently (in v8.31 and before).
Ie. a few stat increases (like working in Nika's clothing store) might happen, even if the npc is given a day-off (eventhough you might not earn any money) and npcs on scavenging duty, will happily perform their job, even if tied up (either on the rack or even in a prison cell) or given a day-off, also Slave Master/Mistress will happily perform their job, if they are currently lavishing in a prison cell (or if they have been given a day-off), a guard captain (even if given a day-off or in bondage) will happily train its guards (eventhough at least the guards should not be bound during the midnight calculations, but they can be trained, if they have been given a day-off), also factory workers will happily work in a workshop (even if strung up on the rack, given a day-off or if they are only able to work in a prison cell and not in the actual workshop). The Janitor will happily increase the workaholic trait (if given a day-off), but will not do so, if he is in bondage during midnight calculations (please note, that during each midnight calculations - at least in the current public version - either all janitors will increase their workaholic trait, if the other conditions are meet, or none will - as the variable $lossroll is used and not _lossroll). Also note, that you will gain money from npcs doing academical work, even if they have been given a day-off (or they are in bondage during midnight calculations), but you will not gain money from npcs working at a vendor or from Gold walkers, if they have been given a day-off¹. This is info for v8.31 (and not for the current alpha version - eventhough I doubt that this has so far been changed for the current alpha and I doubt that it will get changed, unless somehow Grimdark becomes aware of these inconsistencies and considering the chances done from v8.3 to v8.31 I doubt that he will get aware of these inconsistencies by me having posted this info in this thread).
ps. The list is not complete.
¹) Why did I explicitly write, that they are not earning money, if given a day-off, but did neglate to mention what is the case, if they are currently in bondage (ie. have been in a prison cell for a few days or are enjoying a nice bondage session on the rack at midnight)?