You're going to hear varying replies from various people... but I think the general consensus is that valve springs on 500 EFIs are to be placed around the 300 hour mark. Some do them sooner (I did mine this winter @ 250), and some people I know have 450-500 hours on 500EFIs and never touched a bolt on them. All in all I think most people have great success with these motors, and for the nominal amount of money it costs to do a fresh set of valve springs I think it's very cheap insurance.
If the motor has good oil pressure and leaks down well, I wouldn't be scared of it one bit. I'd plan on doing valve springs immediately, and I'm sure it'll need a rebuild in the near future, but as is often the case with these motors/this hobby, it all depends on how the motors/boat were treated before you got it.
Good luck... enjoy.