Honestly it depends on where you're looking to work. Check their policies.
I'm trying to get hired by the US gov, but I'm also just beginning my transition process. I've applied, interviewed, and have a very high chance of getting the job. Now, I had to apply as my 'old' self for obvious legal reasons, plus all my school records, old job records, military records are as old female me.
But the thing is, the US gov has a strict nondiscrimination policy (both for hiring and work) that explicitly includes gender identity. If someone wants to transition in a federal job, they have every right to, including the right to use a chosen name, have proper pronouns used, and have appropriate restroom facilities available. To play it safe, I plan to wait until I've officially gotten the job (fingers crossed), and then I'll let them know. That way I can start work under my new name, people can be made aware of what's going on, and overall I think that will be less weird than if I work there for a few months and THEN start transitioning. It's about as much of a 'fresh start' as I can hope for.
So I guess while I wouldn't let them know at the interview (too risky), if they have a non discrimination policy that includes transgender, I would start transitioning as soon as I was hired.
Having never actually tried this though, I'd love to hear of others who have transitioned on being hired or shortly thereafter.