Until the amount of date gets too large (varies depending on lots of things), I
like to put all my data on one worksheet in one workbook.
I can put indicators on each row for anything that I want to "separate" later.
Then I use data|filter|autofilter to see what I want.
And until the workbook gets too large, I'd keep all my worksheets in a single
workbook.
Obviously, your workbooks will be smaller and quicker to update if you make each
a single sheet workbook with just a little data.
But when the boss comes and asks/tells to give him all the sales figures for the
month of January for the last 3 years, you can go searching through multiple
workbooks/worksheets or just open one workbook and filter by that month.
But like any rule, it was made to be broken. But I'd rather separate my data
when I need it separated than try to find it and combine it when I need it
combined.