Should washer hoses be replaced regularly or only as necessary?
I don't know of any specific schedule, but I can recommend several "best practices":


Buy braided hoses.  These are the metal hoses with an inner
plastic tubing.  They are NOT guaranteed not to fail, but their
failure is much less likely.  Their most vulnerable failure point
would be at the connection to the coupling. 
Inspect the hoses annually.   
Do not allow the hoses to kink - allow plenty of space for their
installation.


Failed "fail proof" hose at the coupling:



You could also buy special high quality ($50) hoses - which frankly if you have a finished basement or your laundry is on an upper floor, is a sound investment in damage prevention.



Perhaps your BEST prevention, however, is an automatic washing machine shut off valve:

 

This device has an attached sensor (pic 2) which you set on the floor by your washing machine.  In the event of a leaky washer OR failed hose, it senses the water and shuts off the flow at the source.  For an under $200 investment, if you're protecting a $20,000 basement or worse, a whole house, not installing this once you know of it's existence would be downright foolish.