Should I place an 'Ukigumo' Japanese maple more in the sun or the shade?
What will help the with the variegation?
Here in Zone 9b I have mine in what I would call light shade - it gets no direct sun but the canopy is not dense. It is reliably variegated. I have heard from others that the variegation can vary from year to year, but I have not noticed that here.
Here's a photo at link below of mine.
Here is a link that might be useful: Acer palmatum 'Ukigumo'
Depends on how much variegation you desire. Ukigumo grown in no direct sunlight here will have a very light coloring - almost completely pink/white with minimal green pigment. The green becomes more prominent the more sun it receives.
It has also been my experience that the more sun this tree receives, the more reversion to solid green foliage occurs.
I have heard the same thing about the sun/green reversion.