I only put "CFI" as in "2785123CFI exp. 03/07".
I asked the examiner about this exact issue after I passed my CFII checkride.
The examiner said there is no particular way it needs to be signed. He said that your certificate number is still only "CFI" even if you have CFII privileges. Therefore, putting "CFI" at the end is sufficient. However, he said some CFII's like to put "CFII" at the end in order to signify that the training came from a licensed CFII, not any old CFI. It's a matter of preference, and both ways are legal.
I only put "CFI" because that's what my certificate has printed on it. The regs say to write your certificate number, so that's what I do. Of course, I don't see anything wrong with "CFII" if it makes an instructor more comfortable for some reason.