From history (WW1) it is known that the English shelled the German trenches non stop for almost the entire war, Before suicidal attacks, After suicidal attacks and often during suicidal attacks (well, you might hit some of them as well!) and the outcome was generally very poor. Even air attcks on dug in targets are usually not effective as anything but a direct hit will usually have no effect. I would say Dig in, Dig Deeper, and when if your losing - change sides, it's easier and it cuts down on the paper work.