I prefer to cut each piece exact before any sewing. For me it is faster and more accurate than having to square up after pressing and sewing.
The way I see it, if you add a bunch of blocks to one strip, then press, then true/trim, if you are trimming anything, it always seemed to be the width of a previous strip and not just the length of the strip just added. Maybe the edge of the block stretched or something. Sewing the pieces cut to length proves the accuracy of your seam allowance. I would teach beginners to do it this way so they master the 1/4" seam allowance.