If you are setting up an array from scratch, then 95% of the time it is a waste of time. Do you feel lucky? There are other ways to test drives to weed out a bad drive before you trust it, so if you use the drive manufacturers utility and do at least 1 full surface scan, then don't bother with preclear.
However, after you have a running array, and want to add a new drive slot, then preclear is a huge time saver. If you don't preclear, unraid will be offline for many hours while it clears the drive to make it match parity.
If you are sure your new drives are good, and keep good backups of your data in case you are wrong about the drives being good, then yes, testing drives before using them is a waste of time in most cases.