Trying it out with B is even more dangerous than doing it with the curtains closed. With the curtains closed, you can feel if you hit anything. With the curtains opened, you can have a lot of bent metal and/or scratched lenses before you know what hit you.
If the lens hits anything, it is likely that it is the internal baffles and not the curtains themselves. That said, I wouldn't risk it. Plenty of nice non-collapsing lenses out there (many of which are identical to the collapsible versions).