why not buy yourself a chop saw? (electric mitre saw) you can pick up a reasonably cheap one, it will be more accurate than a manual mitre saw. i think i know the one you are using, i bought one from b&q and took it back and upgraded to electric. it cost 80 quid but it worked fine. as it will do nice square cuts (if you use it properly!) you will know whether it was the mitre saw or cupping of the wood that was the problem. and yes, cupped wood can throw out the joints and make them curve.