If you feel you are having a bad spell, or at risk of having one, then I would say go for the 20mg, the doctor has effectively cleared this. If he's saying step up in a week or two then it doesn't make a lot of difference to do it a week earlier. If you're worried about this, is it possible for you to ring the surgery and ask for advice over the phone? You may find then that you won't have to go back for another consultation, just pick up a prescription. That's what happens at my surgery anyway.