If you kick out someone close to midnight, the probability is high, that some of their other Pokémon are already kicked out of different gyms and they already got their 50 coins for the day. So in this case it would be more “polite” if you wait till after midnight to kick them out—so they can get 50 coins again.
Though I personally wouldn’t stress so much over it. It’s all part of the game. If you’re not actively griefing, kicking everyone instantly out and hogging all the gyms, it’s okay. (I usually check how long the Pokémon is in the gym. If it is in there for a short time—less than one hour—I let it stay.)
If you kick out someone close to midnight, the probability is high, that some of their other Pokémon are already kicked out of different gyms and they already got their 50 coins for the day. So in this case it would be more “polite” if you wait till after midnight to kick them out—so they can get 50 coins again.
Though I personally wouldn’t stress so much over it. It’s all part of the game. If you’re not actively griefing, kicking everyone instantly out and hogging all the gyms, it’s okay. (I usually check how long the Pokémon is in the gym. If it is in there for a short time—less than one hour—I let it stay.)