I presume you mean the platforms. Like anything, if you don't overdo it you will be fine. It depends on how many you are using at once and how long a period each is running for. If you are only using one or two at a time and disabling them when off screen, I don't think you will see any performance hit. The game I just released uses iTween for many things and you don't see any slowdown caused by it. What I don't like about MoveTowards, is the fact that it feels so stiff.
Lately I have begun using HOTween in place of iTween. It is fundamentally different in the way you integrate it, but I have found it to save a huge amount of performance on mobile. On the mobile game I just released, the main dropping of notes uses HOTween and the performance jump was very noticeable. I would recommend experimenting with that, I may be able to help if you have any questions, as I would like to switch the kit to HOTween eventually anyways.