Should have added, ?20 can be worth it because you are sure the parts of these kits will work together. Occasionally when you buy your components from different sources they will not all work together, so if this is your first build it will give you some peace of mind although I stand to be corrected.
In PC terms, soak testing means having the machine run for, say 24 hours, to flush out any "early life failures" or "out of the box" faults. Generally if it is going to fail it happens earlier rather than later. HOWEVER just leaving it powered on is not enough. The real job uses programs that run the disks, open and close the CD drawers and read/write the media. The machine should also be powered down and up many times and the AC power supplied varied across the advertised voltage and frequency limts with the added benefit of mains carried noise for good measure.
Serious testing (almost never seen for domestic products) does this in an environment chamber that ramps the temperature up and down across a range of humidities too.
As you can imagine, testing life critical parts (aviation, medical, military....) is a science itself.
For your question, ?20 would be value for a reasonable test but a waste of money for a simple "power-on overnight".
If it saves you from setting up a system only to find that the hard drive is a dud, then yes, it is worth it. Computer components can have manufacturing faults, just like anything else, but it's extremely rare.
Compared to the overall cost of a new system the ?20 extra isn't a great deal PROVIDED the soak test involves a 48 hour non-stop run, as DieSse says.