Asking a question like that is like asking a room full of guys what their definition of a hot girl is - the answer will vary from person to person by usage pattern.

There are some generalizations that can be made, such as smaller numbers for smaller files... but there's no cut-and-dry "right" answer. In most cases, I generally suggest trying it both ways and then think to myself "Not that it matters, it's going to end up being a completely subjective/biased answer whatever they come up with because the difference isn't generally noticeable".

IMO: Stick with defaults and be happy, unless you're setting it up for benching or some special application - if so, then you can tweak for that application.