If I bought a metal pickguard, would I still need to shield the back of it?

I appologise in advance if this is a dumb question.
What's the difference between a chickpea and a lentil?
I've never paid to have a lentil on my face!
No such thing as a ligitimate dumb question . I'd say try it out, if you get interference, then shield it if needed.

You shouldn't get any inerference, provided that the pickguard is grounded (and it will be if a pot is grounded). It's like having a metal enclosure for a guitar effect; it shields the circuit from RF and outside noise.