No matter what kind of subfloor you have, use cement board on top. The previous owner of my house installed a ceramic floor in my kitchen before they sold the house. We had the house three years and noticed five tiles around the fridge were cracked. I removed them to replace them and the plywood subfloor was black, wet and rotted. It seems the fridge was leaking (obviously for some time) without our knowledge. The water stayed under the ceramic and rotted the subfloor. So, I had to remove the entire floor and repair the plywood subfloor, then I placed Durarock (spelling) cement board on top.
If the subfloor gets a little soft or warps from water damage, the ceramic will not give, it will crack.
Save yourself a lot of headaches in the future and put down cement board. It costs about $9.00 a sheet and it's well worth it.