What model bike and what kind of tubing? If it's a low-end bike with hi-tensile 1020 tubing, it'll be fine. A couple off millimeters is not a problem, but you want to make it very gradual and blended with no sharp creases. I've even heated up 4130 chromoly to cherry-red and done the dent-with-a-dowel trick. However, I wouldn't do it with any of the higher-strength heat-treated tubing.
I wouldn't bother with aluminium or carbon. Although an out-of-true wheel from having someone crash into me in a rain road-race scraped a nice 3mm gouge out of the left chainstay of my Specialized Allez Epic after 50-miles of road-grit sanding. It's held up for 40k-miles and 15-years since. Then again, I've also snapped the chainstays just behind the BB-shell on a Trek 5000 and a 5500 under normal usage as well. YMMV
Last edited by DannoXYZ; 10-18-09 at 01:55 PM.