I am comfortable accepting fixed bid contracts under a couple of different circumstances: (1) I'm competing against really expensive contracting firms, so I'm charging 2x the rate I'd be comfortable with. There's lots of margin for changing my mind. (2) I'm working with someone for the Nth time, and I know that they're good about being flexible.

But in general, fixed bid contracts are difficult. They incentivize the parties to spend time talking about what the agreed-to scope was, rather than spending time fixing things to however we NOW want it to work.