cement backer board or not?

I want place new tile over existing tile for my hearth. I have a 1920's home, the existing tile cannot be removed because that is all there is there. I am extending the tile out onto the old hardwood about 6". do I need backer board to do this? I don't want to put it down because we upgraded to a sealed gas fireplace and I would have to adjust the height of it if I place the backer board- I would only have to slightly adjust the hieght of the front facing without the backer board. If I don't put the backer down will my new tile crack?