There are times that it just makes sense to press to the light side--especially when it will help to nest the seams. If my light fabric is thinner or a looser weave, I have been known to trim a smidgen more of the dark fabric (or alternately, leave the light fabric seam allowance a little 'fat') so that I won't end up with a dark shadow.
There are no hard and fast rules about pressing or trimming. You have to do what it takes to make the project work.