No, you should only take one spur from a ring final at any given connection point, whether it be an existing accessory or a junction box. If you can find some way of breaking into the ring rather than spurring from it, that would be preferable, second to taking a spur directly from the back of an existing accessory. A good electrician will try to avoid the use of JBs if at all possible, and they certainly shouldn't be installed under the floor or in another inaccessible location if the connection is made via screw terminal.
EDIT: I think I should clarify some terminology here. You mention you are installing 'spurs' - do you mean an electrical accessory containing a fuse and, possibly, a switch? If so, you're probably taking about a Fused Connection Unit, FCU for short.
A spur is a length of cable taken from a point on the ring to an electrical accessory, the said length of cable not directly forming part of the ring circuit.