This made me giggle, lol.
As a Surrogate people do try to 'hero worship' you XD. Very embarrassing sometimes, lol. Honestly. I say there is no bad time to mention it, but just don't mention it to earn praise. Just smile and go, "Yup, I'm a Surro!" (Or in your case a donor, and my future case) if it comes up.
Besides, not mentioning it till it comes up another way is somehow both humbling, and ego boosting. Take pride in what you've done.
Honestly. I think it would totally be sexy to see a guy have a scar and go 'Ooh, how'd you get that', and then him going
I donated to save a life. <<<Hot.
That's my suggestion anyway =P.