A rail fence block is the easiest to figure out assuming you sew a consistent seam. Once all the strips are sewn together and pressed, measure across the strip set. Then cut your blocks to that same measurement so they will be square.
Most of us use a 1/4" seam so each unfinished block is 1/2" bigger than the finished block. In order to make sure, we often square up the block so we have a consistent size.
The 1/4" seam is most important when you have blocks with points such as flying geese blocks. You don't want the points to be chopped off. For most blocks it is important to be consistent.