Certain areas may heat up more in the kiln than others which requires rotating the piece to compensate. Also the temp may vary in different parts of the kiln. These and other variables need to be considered!
Some warpage will often take place even with counterenamel on the back of the piece. There are many variables at work. When it is still molten fresh out of the kiln an old heavy iron may be placed and pushed on the piece to flatten out any warpage that takes place!
Yes unless you go with thick gauge metal to compensate for the different expansion and contraction rates of the enamel and the metal, otherwise the metal may warp and the enamel may pop off! Sometimes depending on what you are trying to do the warpage in the metal may be a good thing but...