I haven't done much glazing other than a few projects in school. It was a real pain for me. I always ended up starting too dark so by the time i started building layers the image looked way too dark.
Photoshop is definitely a great way to test compositions, overlays and color choices. I often wonder what great artists of the past would have been able to accomplish given the tools we have today.
Yes, glazing is a fastidious business. Logically, between glazing, we do a bringing up of the whites and then glaze down, otherwise it always gets too dark and saturated. My students would always try to go too fast with glazing (putting down too much color) and then fight to bring the lightness back. But really it is extremely satisfying and ages beautifully as the layers merge.
I think Leonardo da Vinci would have had a great time with digital cameras and photoshop - he did his best by inventing the projection method with his camera obscura.