Remodeling - When do you change flex hoses / Slipnut Washer

Is there a general rule whenever you are remodeling, removing a fitting or hose - that you use to replace with brand new.
Granted, you pro's may not have a ~history~ of said fitting so you may just do it out of best practice.

I removed some stuff....which all looks fine. Do I replace the slipnut washer on the trap, and new hoses...or just resuse if it don't leak.
If I had to guess, around 8-10 years..

If you weigh the cost of a new flex hoses against the cost of it's failure, it's hardly worth the $10 not replacing the hoses during a remodel. Flex hoses have a plastic or rubber compound core which I don't see lasting forever however I've never had one spontaneously burst on me either.