why preferred way to shield critical signals is by vss and not vdd?

[prashantsid]

(working with layout team in memories..)

why bitlines/wordlines are shielded by vss (most of the times) and not by vdd?

 

[shaiko]

why bitlines/wordlines are shielded by vss (most of the times) and not by vdd?

To me it sounds like one of those "black magic witchcraft" advices.

Just make sure that the traces are short, impedance is matched, no stabs exist and that there're no discontinuities in the reference plane.
Your plane can be:
Vdd, Vcc, Vss, GND...whatever. As long as it's conductive and without discontinuities you'll be fine.