I have always had mine done every year. If the car is used a lot it's not a problem to miss a year, but if not then brake seals do dry out so the brake test is crucial. I had two rear cylinders blow on an MOT as the car had not been used much.
I know its a snapshot once a year but considering what we spend on our Classics, the cost of an MOT is not that much and can you put a cost on your life?
On another point, has anyone had an insurer not pay out when a car has been self MOT'd? From my reading of the exemption rules the car has to be completely stock. Thoughts?