For lots of things it's fine, yes, but for tools I would not try it. My brother (a contractor) had two compressors die from undersized extension cords before learning about properly sizing them. More resistance --> lower voltage --> motor spins slower --> pulls more current --> compressor electronics die.

I'll grant you a contractor's usage of any tool is much higher than a typical homeowner's, there's still no reason not to get the beefier cord. Buy once, cry once.