I have put a satin ribbon around the base of 3 square tiered cakes and secured it with double sided sticky tape but when you look at the front, the ribbon seems to be loose and sagging?

If your putting the ribbon onto a fondant cake, Then you dont need tape at all

Wait for your fondant to firm up first, then take your ribbon, and in the very back, Put a small dab of royal icing. Attach your ribbon, while holding it into place, wrap it taughtly round the cake, not so tight that will cut into the cake, but tight enough that it looks pretty and not loose or saggy, then with another small dab of royal, Attach the other end.

i have know people to use a dainty pin, with a bead on the end to secure it just pushed into the fondant, But in this instance, you will need to be sure that you remove it before the cake is cut, Or you explain to the customer about this.