I agree with your dealer on this one. Alignment can adjust the camber of the tires so that you can corner well without grinding the sidewall.
take a look at this Mazda website example.
http://www.mazda6tech.com/index.php?...id=8&Itemid=50
When you watch F1 racing, you can actually see the front tires being off camber (negative i believe, not neutral) so assist in cornering abilities.
increasing the pressure will not do much to help with the side wall grinding but you can put more weight at higher speeds by increasing tire pressure.