Does leaving an oil painting on canvas unstretched damage it?

I have several oil paintings that are around 3-4 years old on canvas. I have yet to get them stretched and framed. Will leaving them unstretched ruin them or damage them in some way? Some of them have ripples that you can see, but it doesn't bother me, asthetically. I just want to know if I'm doing damage to the paintings and if I should just go ahead and get them framed for the long-run.