If he/she is going to find out anyway, I'd rather it be sooner than later. Once you know for sure they're cool with it, it'll be one less thing to worry about, and moving is stressful enough it is. Disclosing yourself instead of letting rumors do it for you also puts it in the context of "hi, I'm a nice guy and a potential tenant who just happens to be trans". If they hear out from someone else, it may be framed as a "scandalous secret" or even as freakish or disgusting, which, sadly, may color the way your landlord sees it.
I came out to my apartment manager because my lease was up and I needed to re-sign the lease under my new name. She was really cool about it- when we were done with all the paperwork, she shook my hand and said "Nice to "meet" you, Ben."