I always recommend running a Bec where servo performance is important. From what I understand turning the wheels of a crawler on angled rocks can put a high load on a servo. The voltage of a typical internal 5amp Bec is 5-5.5v and can drop significantly under the load of a single servo, especially a stalled servo. With a 10amp Bec set to 6.5v (which a 6.0v servo can take) you won't see much if any voltage drop and will get maximum performance at all times.
Collector1231, While the Traxxas VXL3s esc does support sensored brushless motors it's a well known fact that it doesn't play well with all sensored motors. Are you certain the motor you linked is one it will play nice with?