i think running through the line is pretty important for a few reasons. as someone said earlier, it would allow you to get a more accurate idea of your fitness. would webb's training be different if he ran 3:54 last weekend? (not to say that he would have, i didn't see the race. but it's just an example) also, just knowing you gave it your all is important. then again, if i was doing it for a living then maybe i'd have a different opinion. but i don't. thus, i try to run as best i can, especially when i near the finish.
i think wariner eases up too much. sprinters tend to do it to show how "easy" they were running. but it's the short races in which running through the tape is most imporant. but, if you're going to ease up, you might as well get the crowd excited by showing them some love with some fist pumps. that is one of the things track and field lacks. it's not cocky, it's confident.