My expoerience is almost the exact opposite. Finishing the bonus missions not only yield more experience than the main mission for killing what you have to fight anyway, it is also the fastest way to aquire commendations. I moved off of Tatoonine when the missions were starting to give 5 experience and I abandoned several missions left unfinished. I skipped most of Alderaan past the opening area where I just finished the class quest and moved on. But then I never respond well to political manipulations anyway. So in the end, there was never a need to grind anything.