I always always always make a test block before cutting all the pieces out, to make sure the pattern is right (printing mistakes happen, unfortunately), and to make sure I know how to assemble the block properly.
Once the project is a "go", I usually cut enough pieces for 2-4 blocks at a time, then sew those, then cut some more. (Just a personal preference.)
Depending on the pattern, it takes me just as long, or even a little longer, to cut the pieces out than it does to sew them.