Using more than one conductor in a cable as ground is okay if they are terminated together at both ends.
There is a hazard for ground loops for other configurations. Particularly for multiple cables that don't terminate in the same location on the board.
The key for ground loops is the area of the loop of wire. The larger the loop, the easier it is to introduce a loop current. The more loop current, the larger the voltage difference at various points of the loop. If you have multiple cables, there is both the potential of a larger ground loop current and the voltage difference at different points in the circuit can cause havoc. Thus a ground loop problem. In the same cable, the loop is small. Even if it wasn't, the termination at both ends of the cable limits the effect of a ground loop current.
As SeanB mentioned, minimize the loop area. Also, if you have a high speed differential signal, keep the pair adjacent and twist the pair if possible.