It matters in the first two rounds, which is where all of the best talent is always concentrated. You still have to scout guys, but the odds of finding a future All Star are the greatest in those two rounds. The higher you are, the larger the pool you have to pick from. After that, matters become much more reliant on the scouting department. Obviously having a top pick will help you a bit, but a top 5 pick in baseball doesn't mean anywhere near as much as having a top 5 pick in basketball or football.