It depends. If it is a purchased pattern: 1) Hard to assemble - I cut out of scraps (including old sheetings etc) to test the pattern. 2) Easier to assemble - I cut a few out of the fabric and assemble them to make sure the pattern is true and then I cut out everything and sit and sew everything. I always save borders for last as I sometimes change my mind.
If I created the pattern - I do a test block (out of Christmas fabric - I use the test block to make a potholder and gift it or donate it) and then I cut it all out etc.