Smaller firms may not be able to Transax your cheques. Independent quality certification bodies provide some measure of reassurance. A company that has any or all of the above is more likely to be a company that takes quality seriously, and has a policy in place for it. However, it says nothing about the service and quality of after-sales that you may get. When a company has won several independent awards for their PCs they are more likely to know what they are doing when they are building yours.
Some firms go further.
Always make sure you understand whether the warranty is on-site, at your premises, or RTB, which is Return to Base. Do remember that your warranty becomes virtually worthless if the company that provides it goes bust. If the company you intend buying from has been around a few years there may be a better chance that they'll still be around when you need them in the future. Most new businesses fail within the first few months. Companies that have been accepted for supply of computers to government departments, local councils, hospitals, educational authorities etc may have passed several quality tests to ensure that their computers meet with the respective department's own internal quality standards.