With the churn dash block, there are points that come out to the finished edge, so if you just made your blocks larger and then trimmed them (more than a smidge), you would be cutting off your points when you sewed your block to the next one. It is possible to re-design the sub-units of the block so there is more than 1/4" of fabric from the tips of the points to the edge of the block, but this is a bit of work. (People refer to such blocks as "floating".)
Accurate cutting and piecing can give you good blocks without the need to trim blocks down.