I echo the advice above - your situation will determine the right answer. As an example, I had so many USAir miles, and with the risk of liquidation, the marginal value to me was near zero - so I burned some in what others may determine to be a waste of miles.
For MR, my quant jock analysis resulted in a valuation of 1.2-1.8 cents, depending on my assumptions. I tend to save them for longer stays (marginal cost of nights goes down with longer stays) at higher demand properties, so I personally would use $$$ in this scenarios.
YMMV