Many canileavers are not perfectly straight. Most tonearms can be rotated to adjust for this (azimuth). A jig for proper cartridge alighnment is available from several different manufacturers (Pro-ject comes to mind). This gives you the wizard hat you need to get teh cart "dead on balls accurate" to quote Marisa Tome (see My Cousin Vinny). A quick Google search brought up this website (http://www.users.nac.net/markowitzgd/cartalign.html) that appears to get the basics right.
Good luck!
Good luck!