Determinate tomatoes shouldn't be prunned as they are set to stop growing at a certain point.
On the determinate, which are most tomatoes, I never prune. Only prune off the bottom leaves so they don't touch the soil and cut off any unruley branches that don't want to go in the direction I want them to. More branching and growth mean more flowers and more tomatoes. So why cut them off?
If you have lots of flowers but few tomatoes setting try gently shaking the plants each day to inprove pollination. Otherwise I would say get a good quality tomato fertilizer and give them some food. I start my tomatoes off with compost and then continue feeding with a liquid fertilizer all year long. Personally I would not use Miracle Grow unless the plants really need a lot of nitrogen. I like to use Neptunes Liquid Fish/Seaweed Fertilizer or AlgoFlash.
