Since I am post-op with realistic results, I've also considered this hypothetical. Seeing as how I'm in a LT relationship, it's very hypothetical.
If I were to be having casual sex with someone, I probably wouldn't mention a thing.
If I were to be in a relationship that I felt was going to be long-term, I feel it would need to be discussed. Presumably in most long term relationships, condoms are eventually not worn and/or lack of cum would be noticed. There's typically also conversations about children and while I don't desire any, I'm sure there's dudes that do, so their sterility must be explained, etc.
Also, in years of personal observation, pre- or early- transition FTMs are much more inclined to say they'd be absolutely upfront 100% of the time where post- transition FTMs are more likely to say if someone doesn't need to know, they don't need to know.