Depends on how long you intend to stay on the fast.
Due to monetary reasons I had to use store-bought juices for about half of my 28-day fast which is a long time, and in the end I have to say I started to experience scurvy-like symptoms - for example, my gums and teeth went sore and sensitive.
Pastourised juices often contain added Sugar
which - should we be really strict - should not be a part of a juice fast. Also the juice is dead due to the pastourising - there's nothing living in it. The juice lacks everything you need to nourish your body, you're feeding yourself air, practically. You could end up deficient.
So, in sync with the previous reply.