I usually do my own design so am pretty confident about my sizes, but I have learned that a test block is a good idea. I usually use whatever bits and pieces are floating around for the test.,
If it is a "planned" quilt, with only certain colors, yes I cut all of it first. All colors and all shapes.
For my usual scrappy style, I cut at least each stage at one time. I will do strip units when I can, would rather combine short strips of two colors and then cut down into 2 square units, than cut all individual squares. I usually have a count I want, say 4-6 and cut the strips to that length. Sew, and subcut, and on to the next unit. Same sort of thing if I am making HST, I put them in strips and then cut once they are together, no way am I ever going to cut 2k tiny triangles when only 1k cuts will do!
Yeah, if I mistake it's pretty big, but I can figure out what to do to get around it...