I had not put anything beneath my speakers ever since I owned one. (I put mine on a shelf for all my audio-related tools made of wood.) I don't think I've ever used them with any form of cushion. Is the sound different? What did your friend tell you?
It depends how big the speaker is and how loud you turn the volume up. I would go for the foam if your speaker is moving when its on and a moving speaker is usually the case when you turn the volume so loud.