I have a Graduated ND filter that screws on and has an outer ring that turns to place the ND section in the area you wish.
Graduated NDs are not the same as a variable/fader NDs.
Is stacking still a huge issue if you use stepping rings? .........
Is stacking still a huge issue if you use stepping rings? .........
The use of a step ring is not relevant. 3 pieces of glass is 3 pieces of glass. The only thing you may avoid is vignetting.
Depending on what you are shooting and metering off of without the filter, the cast could be more magenta then blue and also depending on the ambient light available and the length of the exposure. This is the same with all ND filters I have, so it's not something I even consider correcting with WB before or as I am shooting.
Sorry, Vignetting was the big deal I was referring to; you say "may". As in it doesn't always prevent vignetting?
Using larger filters on a step right MAY prevent vignetting. Without any specifics (thickness of filters, step ring sizes, how may you'll stack, and aperture and FOV of the lens you're using) it's impossible to make a blanket statement.
I see your point, I'm not sure how often I would plan on stacking them anyway, but its good to know it isn't a guarantee! Nothing worse than relying on something to work and finding out it doesn't!