What would you say is THE MOST important factor to good imaging?

Big part is the recording. Other components having superior phase characteristics.

a good bag of ganja
mexicola gold
and some pink floyd
yea

Since you have to get everything right , hard to say .
Try toe in every 1/ 8th inch over a week or 2 .

You don’t have to do 1/8" increments.

Image placement:
Toe out too far you start to get just a softened center image, and nothing either side of that center image.
Toe in too far and you get nothing outside the speakers, and a strong center image.

Depth perspective:
Comes from how good your speakers are, what’s between them (hopefully nothing), how far from the back wall they are

It’s all what your ear/eyes are hearing and seeing, and use a good disc that has known great imaging spread and depth. Or get a simple setup disc like the Chesky JD37, whatever you do don’t play from track 28 on wards.

Cheers George

Of course you don't have to but in last  50 years has worked best for me.
Of course that's only on acoustic music  aka jazz and classical .
With rock why bother .