1 to 5 is .7 orders of magnitude; 1 to 2 is 0.3; both are roughly equidistant from half an order of magnitude.

Clearly we need $1, $3, $9, $27 bills ;)

An interesting thing to think about is-- assuming the distribution of transaction values follows a power law-- where's the point of diminishing returns on having more denominations? I know this isn't well-posed; we'd also have to quantify the downside in having more denominations.