If you're not experiencing crashes/bsod's due to hardware problems you're probably fine. Considering you've had it up and running for a year I'd say you're ok. Stress testing is for finding weaknesses in configuration/hardware that could potentially lead to system instability. Usually used for overclocking because it's pushing components in ways they weren't made to be run straight out of the box and adjustments need to be made to regain stability and make sure temps are within reason.
Edit: No need to be concerned about stability if you're not experiencing crashes. It either runs or crashes, blue screens, turns off. The idea of stress testing is if changes are made, since weakness can show up under load even if it seems fine at idle or web browsing. You don't want to be in the middle of a game after overclocking just to find out the cpu, gpu or ram are unstable and have it keep crashing a few minutes into a game or during video editing, that sort of thing.
The other purpose is temperature monitoring, making sure that changes like an overclock aren't causing temps that are too high - again because default operation of components was changed. It can be a good idea to monitor temps every once in awhile anyway using something like realtemp which is a small program. That way you can be sure it's not constantly running slightly overheated.