When writing is it correct to use numerals (e.g. 1,2,3,) or spell out the number (e.g. one,two,three)?
While there are indeed no concrete rules for general usage, I would argue that the general rule is to spell out a numeral by default. Consider the following sentence as an example of sloppy usage:
He had 3 dollars.
It might be technically correct, but it looks incredibly amateurish.
Though some may disagree, I think one might argue that the importance of spelling out a numeral is roughly correlated with a numeral's size. Consider this sentence:
He had 113 dollars.
That doesn't look quite as bad as the first sentence, and who wants to write "one hundred thirteen dollars," anyway?
On the other hand, when writing something that includes a lot of numerals (especially if they're big), written words can become confusing. For example, let's say you're making comparisons between the number of species in various animal families. Consider the following:
There are thirty-six species of felids, thirty-seven species of canids and one hundred seventeen species of mustelids.
This is much easier to read:
There are 36 species of felids, 37 species of canids and 117 species of mustelids.
However, the first sentence isn't too bad if it's accompanied by tabular data:
Felidae | Canidae | Mustelidae
36 | 37 | 117