Generally you just need to keep any & all parts of the interior above freezing. However, local factors can trump any general advice. For example, are there any semi-exposed water lines, e.g. in a crawl space or attic? Do outside temps get severely cold, e.g. 0 F., or just "below freezing"? I s humidity a problem? Any of those things might dictate a higher thermostat setting or "winterization" ala Ernie's method.
Most thermostats don't have a setting below 50-55, so that may be the limiting factor.