To help streamline the decision process, the following equation assumes you will get double miles on your entire tax payment:

P = payment
W = worth of mile

0.0249*P = W*2P, which reduces to:

0.0249 = 2W

0.01245 = W

So if you value a mile at more than 1.2 cents, charging is worth it.

A previous post valued 10,000 miles at $125. This value is very low if using SPG. I just paid 60K points for six nights at a St. Regis (12K/night plus one free). That works out to 10K/night and the rooms, with tax, were around $800.