I would think the best way to figure it would be as a percent of your average daily sales, perhaps around 25% to 30%.
Obviously, how much of that would need to be change (and what variety of change you would need) depends on if you are including tax or not in your prices, and whether all your items (with tax) end in multiples of 25 cents. As far as the bills go, I would say about 2 to 2 1/2 times as many fives as tens, and about 2 to 3 times as many ones as fives, unless most of your prices including drinks, etc., (after tax) are multiples of $5, in which case you could take less ones.
How do these figures compare with what you have been taking recently? There will always be times, no matter how many smaller bills you take, it seems that almost everybody has only a twenty.