Is it okay to say 我下一下 when e.g. getting off a bus? Once I saw a woman get off a bus and I thought she said 我下一下 to inform the driver that she was getting off.  However, I mentioned that to a Chinese friend, and he seemed to think it's weird, so it's possible I misheard.

It's normal to say things like 我坐一下 or 我百度一下, where 一下 emphasizes that it would be quick; there are other examples in the Chinese Grammar Wiki article Verbing briefly with "yixia" (see also the question When to use &quot;一下儿&quot;).  So I don't see any reason why 我下一下 would be improper.

Question: Is it okay to say 我下一下 when e.g. getting off a bus?
I think it's ("60%" OK, "40%" wired ) to say "我下一下" to driver when you are getting off from a bus. Chinese people know what you mean, but few people would say like this. 

In most cases, people would say "您好，我想下车".  it's quit nature. 

your examples of "我坐一下", "我百度一下",  that's very OK, here "一下" means "try once"