I've found that it's really helpful to keep a stippling sample in easy view next to my machine when I'm stippling a quilt, especially if the size of the stipple is not my norm. It's easy to lose perspective and fall into your natural stippling size without realizing it. I think everyone has their own unique stipple size that feels right to them.
I had stitched out some samples in a variety of densities a while back. They are also helpful for showing customers vs. simply telling them you're planning to do a "large" stipple or a "dense" stipple.
Still, it's a challenge to end up with the same size stipple the whole way through a quilt!