Is it best to have the single spacer and a wheel dish to maintain shifting accuracy?
Is dishing a big job,does it require new spokes?

I’ve got both.
Single spacer and wheel redish or two spacers and adjust the rear mech.
There’s not much in it either way.
Redish is easy but so is re adjusting the rear mech.
From memory it was around one full turn on the spoke either way to redish.
Depending on what spacer kit you use it may be easier for wheel removal/fitting with a single spacer on the brake side.

Redish isn’t a big job (15 mins with a homemade dishing tool) but you don’t want to be doing it forwards and backwards. There is a formula for how many turns as well…

As singlespeedstu says a single spacer might be better.. certainly less to lose.

I swap wheels across bikes so redish isn’t a real option.

I found that with both spacers on one side, the wheel wouldn’t rotate as there’s an annoying preload adjusty thing that just (by a fraction of a mm) failed to clear the disk brake bolts.

So spacer on each side.

Pro tip: get a spare set of spacers.