On the real thing, once you reach the end of a section, you can go back and 1) review specific questions, 2) review all marked questions, or 3) review all incomplete questions. So you don't necessarily have to mark a question to go back and review it, if you can remember the exact question(s) you need to go back and review (I wouldn't recommend this under actual test-taking conditions), but I'd just mark anything that you think you might even have the slight possibility of wanting to review.
Achiever modeled this "feature" of the actual DAT exactly... not sure how TopScore is set-up.