Marcus, always let your alignment gauge be your point of reference. Remember, you are establishing tangency between your cartridge and the record surface, not the tonearm and the record surface. Most alignment devices presently available are establihing the ultimate point of tangency at approx. the middle of the record playing suface.
In some cases, the cartridge may appear visually to be misaligned relative to the headshell portion of the tonearm. I presently use a VPI/RB900/CLEARAUDIO combination and the cartridge is not perfectly aligned in the headshell.
This will always be somewhat of an issue with a fixed point tonearm as perfect tangency will only be had at one small point of the record surface. On average, there is approx. 3-5 degrees of tangential tolerance since a fixed point tonearm cannot track tangentially accross the complete record surface.
Also, you may want to check out some of the posts regarding setting up your RB300. There are some interesting tricks regarding setting tracking weight and anti-skate (or lack of).