Hard Disk Monitoring

I have purchased several new hard disks and looking to install them in my NAS.

The FAQ's state the following steps to detect and repair any hard disk problems:
1) Disk -> Short self test
2) Disk -> Extended self test
3) Disk -> Surface test -> Read test
4) Disk -> Surface test -> Reinitialize disk surface

If the first two steps does not detect any problems, will it be safe to use in the NAS, or should I continue and perform Step 3?

If the first two shows no errors, then a complete surface scan (by the Extended self test) already completed, so you may use in the NAS.
These tests usually fail (usually quickly) upon any issue (rarely even on a perfect drive, as disk controller/motherboard chipset may also affect the internal self test outcome), so if they complete - then in most cases the drive is really perfect.

To be safe, personally I'd also use step 3, the Disk -> Surface test -> Read test, because it reports more details than a simple "pass" or "fail" status: this read test shows/verifies the transfer rate too, so reports if there are slower areas than expected (with darker green blocks). This is not reported by the internal hardware self tests.

So maybe if you prefer to save time and just run minimal testing, personally I'd say to use

1) Disk -> Short self test (to check if the disk components are OK in general)
and
3) Disk -> Surface test -> Read test (to verify all sectors and report possible errors / retries / slower areas too - or confirm that the drive is really perfect.