Typically as a new agent you are a buyer's agent a majority of the time anyway, then as you get a history in your marketplace, you get heavier on the listing side. I wouldn't pigeon hole yourself as only a buyer's agent, however, if you have the opportunity to work as a buyer's agent for a producer, go for it.
I have a buyer's agent that drives my buyers all over, I do the listings, and he gets half my buyer's side deals. It works great for us.