W is no big deal, because different schools have different guidelines. I know at my school you get a W if you drop a class after 3 weeks. That's a really short time, most classes don't even have an exam within that time. I know I got caught in a bind a couple times, once I was switching majors and another time I decided I didn't like the course. As long as you don't have a bunch of W's you should be fine. I was never even asked about it in all my interviews.