You dont absolutely HAVE to farm at the bandit den at lvl 16 with a bow. Its just a great place to farm sp because of the spawn rate. There are other places to farm sp, and you can even farm the entire way up to cap. In my opinion, since you are a blader, I wouldnt waste any sp at all on other masteries except your desired ones.
You should check on the character builder
http://nivlam.com/char/ to find out how much sp you need. Good luck with this char, its going to take a long time.