Humm... this is really odd and I do hope it was just a misinformation because, unless my infos are wrong, the folic or folinic acid really are recommended especially with the low dose methotrexate. It does reduce the adverse effects and it helps preventing the need of discontinuation of the treatment. It helps regulating the liver enzymes level, nausea, tiredness, ulcers,etc.. It also reduces the artherosclerotic effect of the MTX.
I was advised not to take it on the day of the injection but the remaining of the week, I was told to take some folic acid. The dosage seems to change from doctor to doctor. Some will suggest a daily basis and other will suggest a big dose once a week.
So yeah, I don't want to tell you not to listen to your doctor, but if the response is again that you should not take folic acid... I would at the least have a discussion about it with your pharmacist or GP as it seems a little weird...
Sometimes people don't know... I went to the ER once and the nurse asked me if I was pregnant right after I told her I was on MTX. She asked because of the folic acid... I told that anecdote to my friends (well my nurses friend, I have a few) to see if I was supposed to be scandalized or not by this, and well their eyes told me they knew it was highly teratogenic...