From personal experience I have noticed the power of vibrations are not affected by the position your body is in.
Sometimes when I am doing long sets of vibrations or mantra's I actually switch positions, from standing to sitting to laying and so on, because it starts to feel uncomfortable to stay in a certain position for too long.
All positions your body is in affects the floww of the bodily fluids in different ways and it might feel better to stay in some position over another, but generally the power my vibrations produce don't seem to be affected.
A meditative pose while sitting, such as the pose the Shiva statue is depicted to take seems very powerful when meditating, but standing and even laying can be just as effective I have noticed, however with laying you can loose focus faster and drop deeper in trance or even fall asleep accidentally, so in some ways it seems energy get's roused up better when one is actually in an active position, such as a meditaive pose, keeping the back straight, chest puffed out and body in a strong posture, then focussing on the energy you are working with intensely. This will in most cases greatly increase the amount of energy you feel as opposed to laying down.
The power of your vibrations has more to do with your level of focus, the direction of your intent, the clearness of your mind and the awareness of the energy you are working with. If laying down makes it easier for you to focus then you can perform the RTR's that way as for as I know.