I don't think there are any "official" rule as such. That said, for companies or administration sending letters, it is very common to write recipient name and address in top right corner so that once folded (commonly in 3 parts, accordeon style) this info can show through the transparent window of the envelopes commonly used in that case. Hence you don't need to print it on the envelope and don't take the risk of putting letter for A in the envelope aimed for B.
Other practices are more common sense or generally used, like putting the signature(s) at the bottom or the date (+city) on top left. There could also be internal rules like "at company X we always place the signature on the left".