It comes down to common sense & experience.
Would you reuse a seal when there is no urgent need for the aircraft back into service? No way.
However, when you are in a situation either away from your main base or out station, would you want to lose a flight due to a problem that can be solved? In the hyd filter case, firstly you wouldn't disturb it until you could identify you had spares, but if you did need to refit the old one, at least you could inspect & leak test the system to prove it is safe & functional. Even fitting a new seal is no guarantee of a good seal, manufacture error or damage whilst fitting ( you DO inspect new seals & back up ring before fitting don't you?
) can easily cause problems.
Are you going to ground a SAR helicopter or HEMS machine that is urgently needed due to not having a seal for the mag chip detectors, or are you going to leak check it then make a decision?
Are you going to stop the C-130 flying that is air dropping water ammo & rations to a forward operating base because the Lp oil filter on one engine needing to be checked ( very common spurious fault due to cold oil popping the button!)? Again, common sense dictates that you refit the old seal that is perfectly serviceable & leak test, then make a decision.
Simple blanket statements saying "all seals MUST be replaced" simply don't work in the real world I'm afraid. Part of being an LAE is applying common sense & saying no when it needs to be said, and getting the aircraft flying safely when possible , in my humble opinion.
I am also a big fan of the "would I fly on it" test!