There are plenty of us out here who read, correct, and edit as we scan. Perhaps I am still too new at this (have been scanning for just under a year), but I cannot imagine scanning a book without checking it for completeness, that each page is readable, and all the rest. Sure, it slows the process a whole lot; but if I am scanning with the primary intent of submitting, I feel a commitment to do that.
Getting back to the initial question, I think that the answer is "it all depends." While I am not going to immediately validate what I scanned -- it doesn't hurt to have someone else taking a 2nd peak -- I have no problem with someone who has done all the necessary work to validate their own submissions. Validation doesn't guarantee anything for any specific validation is only as good as the validator happens to do it; and even the best and most thorough validator can miss something as they are human.