Always use a consistent seam allowance. The rule of thumb is a ¼”.
You can cut 3 pieces of fabric at 1-½” wide and sew them together to test your seam allowance. The finished piece will be 3-½” wide if your seam allowances are truly ¼”.
And there is no ‘One Right Way’ to make a quilt. Use the marks on the plate under the foot, a quarter-inch foot, tape, or some other mark or tool to help keep your seam allowance consistent. Just remember that if you are using a pattern, read what seam allowance the pattern designer used. If you use a different seam allowance, your quilt will be a different size – which may require more or less fabric than indicated on the pattern.