I always about a 1/4-1/2 yard more of each color than I think I will need so that I don't stress about miscuts.
When I have cut every thing first, I've liked it. However, I sometimes either misread or miscut pieces so I usually hesitate to cut everything at once. I generally take it one section or one block series at a time and cut just what I need for that part of the project. That gives me some leeway in correcting my cuts or changing up the fabric if I don't like the outcome.
That said, I have a friend who is much more anal than myself and she is very methodical. She cuts everything first and spends a lot of time laying out fabrics and making sure her cuts are perfect. She labels and organizes everything and when she sits down to sew, she is very efficient. That works for her and she does beautiful work.
I am not that efficient and enjoy the process and the ability to be creative and switch out fabrics throughout the project. I like doing my own thing so, figure out what works for you... .