Hi Katharine,
First of all good luck with the TKR's. I know we are all different but I have never looked back since mine. It's hard work at first as you must do the exercises, and it's not comfortable BUT so worth the effort.
My rheummy and my surgeon stopped the mtx 2 weeks prior to the ops (and aso when I had my right hip replaced) so I would say you are doing the right thing. This is really a precaution. As you know, the body doesn't repair itself too well when it is full of mtx so, of you were to contract an infection, it would take much, much longer to get rid of.
Take care and please let us know how it goes.
Love Jeanxxx
PS I thought I'd let you know that I sailed through all the ops, despite being much overweight and suffering from high BP, so try not to worry.