mia, I think the time of year matters because this isn't mainly a question of accumulating more miles or points--or at least that's not what I was getting at. (Can't speak for others.) If, as the end of the year approaches, I don't have enough spending--such as on the BA card for the perk I mentioned--I might well reach that plateau through the tax payment. I'd be reluctant to do that earlier in the year, because at that point I wouldn't know whether I'd need that extra spending (on taxes) to reach the plateau.

I'm not as sure about susiesan's point, but I guess she means that if the convenience fee is deductible then using the CC for taxes in effect costs less.