If you want to optimize for some outcome (renting cars at a known average price, with some known average penalty for promising cars you don't have), you can just directly optimize for it.

But if you just want to get a picture of what's going on, there aren't going to be any non-arbitrary tests. Comparing the standard deviation to some scale of interest is just a useful piece of information people use to understand the problem. Feel free to set any arbitrary boundaries (or less arbitrary but still not optimal, e.g. "six sigma" business practices) you want.