When sewing by machine, you are cautioned to use a scant quarter of an inch seam allowance. This is becuase the double threads used in the sewing machine’s lock stitch (as opposed to the running stitch used in hand piecing) take up more space than the hand stitching. A 1/4 patchwork foot is indispensible in achieving a perfect scant quarter inch, but it does take practice.
Many newer machines have a guage for a scant quarter inch built in. In order to check, take a piece of paper, and feed it into your machine using the edge of the presser foot as a guide. That is usually 1/4″. Take out the piece of paper and measure it. It should be just slightly 1/4″ (not quite) seam allowance. If it isn’t, adjust by moving the needle and testing again until you get the appropriate amount.
Sometimes using seam guides which can be screwed into your machine can also help. The fabric is butted up to the edge of the metal bar in order to help you maintain the appropriate seam allowance. Magnetic seam guides are also available, but I find that they are too easily moved by accident.