Is a <transmission> tag necessary for non-motorized joints ?

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I'm working on the Gazebo model for a robot arm which consists of a couple servos with links attached to the servo horns at off-center joints, which drive the arm, which itself is mostly a series of "passive"(un-motorized) joints with some end-effector attached at the end. Do I need a <transmission> tag for each joint, even if doesn't have an actuator ?

Most of the "robot arm" tutorials and examples I've found in code or online seem to assume that there will be stepper or servo or some sort of motor in each joint of the arm. What would be the correct way to specify something like, say, a scissor-lift ?

This is being done in Gazebo 7.