If you're trying to fill a 5G keg I understand the appeal. I brew all my 5G batches in a 10G pot using 7.75 gallons as starting volume, 6 gallons at the end of the boil, 5.5 to the fermentor and 5 to the keg.
Yes, I'm wasting wort and beer at each stage, but that's the joy of homebrewing. I don't care if I dump half a gallon of wort and trub on my lawn at the end. I know that stuff probably wouldn't have made my beer better anyway.
If you're not kegging and don't plan to be, then I agree with the above. If you're bottling, there's nothing magical about 5G batches.
Regardless, you'll never be sorry you have a larger pot.