Rough rule of thumb for when to use $N$ and when to use $N-1$ when calculating variance.

Question

I don't really understand the whole concept of estimators and biased/unbiased etc with regards to calculating the standard deviation.

Is there a rough rule of thumb that I can use to know when I should use one over the other ?

I mean does it depend on the magnitude of $N$ or the nature of the data or what ?