Some brands of hole saws are less accurate than others.

Black and Decker seem to be oriented toward woodworking and aren't too accurate, but I've used them with success to cut 3/4 & 1" holes in .120 frame walls.

Try either Starratt or Milwaukee hole saws.
They run fairly accurate all things considered.
(Either brand of hole saw fit's either arbor and I think Starrat's hole saws are made by Milwaukee.)
They're a bi-metal hole saw and I had no problems in cutting reasonably accurate holes in .063 thick stainless.

Use slow speed and a solid setup - like in well clamped etc.

It may help to drill 1/4" holes just to the inside of where the instrument hole will be cut. That gives the cuttings someplace to go. An especially important part when hole sawing 1/4" or thicker steel or aluminum.

You may want to make a jig when you lay out your dash.
I used an aluminum yardstick for my 32's dash to cut the dash proper and later on used the jig to lay out the holes for the engine turned stainless piece that went between dash and stainless.
The holes in both dash and stainless were a perfect match.