Generally you want to press towards the darker fabric if possible so that the darker fabric won't shadow through the light fabric.
Pressing to the outside, inside, it really depends how your blocks will go together. For example, if you are doing a 9 patch square, you would want to press the top row let's say to the outside. The middle row to the inside. The bottom row to the outside.
This way, as you sew the rows together, your seams will nest together nicely and not be bulky. Make sense?