I believe that most cartridge manufacturers assume that the tonearm is parallel to the record surface when manufacturing their cartridge. However, there is no reason why a partiuclar cartridge might have a geometry which requires that the tomearm be slightly off the parallel in either direction in order to get the appropriate angle. I would start with whatever the particular cartridge manufacturer suggests and then adjust by "ear" from that starting point.
Why adjust from the angle the cartridge manufacturer suggests? Well, keep in mind that even if the angle is perfect for the tonearm/cartridge combo which you have, it may be off if you put a different record on. Not all mastering studios cut their records at exactly the same angle, although in theory they should be doing so. The proper VTA for playback is determined by the angle of the cutting stylus, not by the cartridge manufacturer. If you want to be really anal about it, you would have to adjust the angle for each record you play. Life is too short for that.
Why adjust from the angle the cartridge manufacturer suggests? Well, keep in mind that even if the angle is perfect for the tonearm/cartridge combo which you have, it may be off if you put a different record on. Not all mastering studios cut their records at exactly the same angle, although in theory they should be doing so. The proper VTA for playback is determined by the angle of the cutting stylus, not by the cartridge manufacturer. If you want to be really anal about it, you would have to adjust the angle for each record you play. Life is too short for that.