I've never gone off of MTX per my rheumies advice. However, yes I've stopped taking it once or twice because I myself didn't believe I had it, and was tired of the MTX, plus didn't think it was helping all that much..was I ever wrong. I did confess to my rheumy, who of course concurred it was a bad choice to stop it, asked how I felt, and was not surprised I felt so hideous.
I've never heard of this approach before with someone who is sero-negative. Is your rheumy planning on checking your sed rate etc before and after to get some idea of your inflammation level? Gosh, I'm sorry, I wouldn't like this approach, and would like to say it may be in your best interest to search for a new doctor. One that is willing to treat your symptom based, sero-negative RA.