Although I'm still on waitlists, my situation isn't pretty... and it's partly due to withdrawals. My situation is a little unique, because my undergraduate school doesn't list W's on the official transcript. Being the Pollyanna that I am, I went ahead and put them on my AMCAS. Well... during processing, they magically changed from W's to F's. I spent months trying to get them changed back or removed... but, AMCAS wouldn't do it, mostly because they didn't appear on my official transcript. I got letters from the professors saying I had not flunked, and it still didn't help. It was a nightmare, and it ended up lowering my science GPA by about .3 points and delaying my secondaries two months.
My advice: Don't withdraw from anything. If you're serious about medical school as a college sophomore, take the easiest science classes you can find junior year to boost up that GPA. Take summer classes at community college if you have to. Remember, Geology for English Majors is counted in your science GPA just as much as Biochemistry.
