Is it harmful? Not to your phone. Maybe to your patience or your sanity. There are certain times my wife knows that I'm off limits for anything less dire than the house burning down. Income tax time is one of them. Having to reinstall all my apps and data is another. I wouldn't do a factory reset without a working nandroid backup unless that was the only possible way to get the phone working - almost anything else is preferable. (Even with a working nandroid backup, I'd rather not do a factory reset. Clear the cache. Clear the Dalvik cache. If you have to do a factory reset more than once or twice, you aren't learning what you're doing wrong. (I think I've done 2 factory resets. One was to see exactly what did and didn't get reset, so it was an experiment. The other was an attempt to get a brick working - that turned out to be bricked because of bad hardware.)