What's the grommet tool for? IMHO, grommets should be avoided as they tend to tear out of things.
Knotty's right - grommets aren't as good as webbing loops with reinforcing patches. However, I have made many poncho tarptents with "grommets" at the corners. I sewed a 2" x 2" piece of heavy webbing to the tarp and used my trusty soldering iron to melt a small hole through it. Haven't had any problems with these, but for a larger tarp webbing loops might be better.