Ficus elastica is one of the most phototropic plants. I can see this in my Ficus elastica also and one I had years ago did exactly the same. What Will suggested is a good plan, turning your plant a quarter each time you water it. This way the cells will be getting the same amount of light on all sides and the "dark" side that is now overgrowing the one that gets the most light will be given more light, thus stopping the overgrowing and tipping the lighted side. Both sides will begin to grow equally, not lopsided. I have started to do the same with mine.
Our fear that it won't get the light it needs is not a reality based one, it will still get the light it needs, just giving it the chance to get it on all sides. Moreover, you won't risk the top of the plant getting so lopsided that it collapses on you some day.
*Here is a photo of mine, you can see it had the same "leaning towards the light source" effect, but I have since been turning it and has improved a bit.
