I've been doing some research on interfacings and ran across this info at Margaret Islander's Sewing System website and thought I would pass this great suggestion along to you all.
I, also, have found that leaving lightweight interfacing in the seam lines rather than trimming off the seams before fusing by iron, helps the seam look better. You will normally read how this adds bulk to the seams but I find that is no longer true with the advent of new interfacings. My seams are nice and crisp and easier to sew in most cases.
I do not do this with heavier fabrics though as it will make the seam even bulkier.
Here's the quote for Margaret Islander...
" In many cases (more often than not) interfacing should not be trimmed from the seam allowance of the pattern pieces. The interfacing gives a more even surface to the piece, especially when topstitching will be added. Exceptions occur when it is determined that the seam will not lie flat with the extra bulk. This is most often the case with heavy fabrics like corduroy and wool coating or when a seamed area includes more than two layers of fabric."