Are the drives you are using new or have they been in-use?
If they have been in-use for a while (maybe over 3000-4000 hours), then you could tell yourself a SMART Long test is good enough. However many of us like to run Badblock on the drive in destructive mode to ensure every location can be written to and read back, mostly to identify infant mortality. Plus Badblock does this with a few different patterns (4 I think it was). And yes, it will take a bit of time to run these tests.
If you plan to place valuable data on the NAS, this is a wise thing to do. @Dan is not steering you in wrong.