Andy, there are two different "grounds" in turntables.
The ones on the RCAs are the SIGNAL "negatives". These go back to the cartridge. As the needle moves about in the groves, it is attatched to a cantilever which is in turn attatched to either magnets or coils. As the magnets or coils are moved about. They are surrounded by either magnets or coils, whatever the opposite is of what is on the cantilever. This is how electricity is generated. You will need to have separate positive and negative leads for each cartridge channel.
The ground lead (usually a separate green wire) is attatched to either the tone arm shell or base plate of the turntable. these are use to keep the refernece ground of the playback system at the same level. Withou the reference ground wire attatched there can be a gooly amount of 60 cycle hum pick up.
This has nothing to do woth the signal negatives coming off of your phono cartridge. Now some phono preamps DO connect the signal negatives to earth ground. Some do not.