Spell out reasonable numbers.
This is the rule with a bazillion exceptions.
Spell out reasonable numbers, such as ten, one hundred, a million. Please do not spell out 1,345,456. That is what is meant by unreasonable. That will probably take up about three lines on a typical paper, and it can sometimes appear to be padding in order to reach your assigned word count.
Bring on the exceptions. The following can be kept as numbers.
Temperature: Sorry I'm not sure how to get a superscript onto this blog right now.
More later. Can't think of the rest right now.