cavity shielding

Is it necessary to shield the pickup cavities with shielding tape
if using noiseless pickups (single coils)?

Not as essential, but yes. Shielding helps keep noise out of the other parts of the wiring too.

so where else on the guitar should be shielded?
pickup cavities, back of scratchplate... anywhere else?

Any control cavities or jack cavities too.

thanks alan... and what about the back of the guitar
where the trem block and springs are?

No - the only electrical thing in the trem cavity is an earth wire.