Ask your insurance company, local fire marshall, and county code department. Regulations vary by state and county, and if you want your insurance if you have insurance, to pay out in case of a fire, you need to keep them happy and have any needed permits and inspection certificates on file. Aside from that, we always used to put them up against a side or back wall, never worried about it. Makes a difference if it's double walled or just a plain old single wall tank too.