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.