The price difference between #6 and #8 is trivial compared to replacing the entire line with #6 if you decide to upgrade to a 50A circuit. I generally overprovision things like this. Most of the time, I don't end up needing it.
However, voltage drop is also a concern. You might have a short run, but the thicker gauge might save a volt or two. With a 32A current, this can save you 30-60W of dissipation. This sort of thing can actually save you money. If you used it for 4 hours every day, the ROI on the thicker gauge would be about 2 years (highly dependent on electrical rates).