Thank you everyone for your thoughtful replies!
Some of you have referred to scenarios that involve daily radiographic imaging and CBCT. In these situations, I agree that skin markings are not necessary because the surface guidance is ultimately confirmed (or overridden) by radiographic imaging every day.
My main concern is for patients who do not get radiographic imaging on a daily basis. I think Ellen gave a great answer for this one, a combination of indexing and a quick SSD verification makes a great redundant positioning check in addition to SGRT.