It depends on the food. I love pre-chopping smoothie ingredients and freezing them in little sandwich bags so that in the morning I can just pop them in the blender, add a little milk, and I'm good to go with a nice cold smoothie.
I'm really trying to get a bit healthier with our meals so I'm starting to pre-chop a lot of stuff. Last night I was chopping pumpkin for a salad so I chopped the whole thing so I have no excuse not to put some in the casserole tonight...I did end up with orange fingers though! I'm also going to pre-chop ingredients for omelettes so that I have stuff prepared for a very healthy breakfast or quick and easy dinner.
Sometimes . It depends on the food and when I am going to cook it. If I am going to cook it at a time when I know it will be late or I will be in a rush, then I do that beforehand.
I'm fairly certain that food will retain it's vitamins better if not stored cut...however if pre-chopping means that you will get more vegies in it probably balances out.