Sometime I use a razor blade to open seams. It makes me feel a bit bad ass.
The short time I spent working at a mall tailor shop taught me a lot of sewing tricks to increase my efficiency. Using a razor blade to open seams was one of them. This works best on long, straight seams or hems. Starting off with a seam ripper is the best way to ensure you don't cut your fabric. Once you have a hole about an inch long, then go in with the razor.
The biggest trick to this is to make sure your fabric is held taunt! Brace the bottom of your seam between your hip and sewing table if you can, or use your big toe to hold the other end of a hem. If it's not held tight, you are likely to cut a hole in your fabric, and no one wants that.
Sometimes I use the blade in this little holder that came with it. It's meant to scrape paint off, but this helps me with my metal aversion. When I need more precision I need to ditch the holder though.