Exactly. I've always bounced between OSes over my career. Switching between them is not that hard at all - just need to mentally page in the key modifiers, shell commands, and other little idioms and pretty quickly I'm running along working.

In my experience I've found inflexibility with respect to OS to be a warning sign that usually means someone isn't going to work well long term as a flexible, open-minded team member. Signs of that at interview time usually push me pretty quickly in a 'no' direction.