In my experience, bib straps do fit differently - some are tighter than others despite the waist size being the same. And I haven't had any of my bib straps stretch much if at all. Maybe my Sugoi RS's have stretched a small amount from when they were new (they are starting to fray they've been ridden so much...), but my Giordana's don't seem to have changed a bit. Not sure what yours will do. I suspect the only way you'll ever find out for sure if they'll stretch out is ride them until they fall apart.

And of course there's the possiblitity that if you've never worn bibs, you think they're just a bit tight, and the first time you ask another cyclist about them you get "My God, those are so tight I'm surprised your voice doesn't sound like Mickey Mouse" or "Wow, those are so loose you could carry a basketball in there." Or both.

FWIW, I like my straps tight and pulling the chamois up because my leg motion when pedaling seems to pull the chamois down some, which can open up gaps between my skin and the chamois and cause saddle sores.