The advice I’ve heard in the general contractor world, is fixed bid only works if you charge appropriately, and are very quick with change orders. As soon as it looks like the scope is changing, be it a request from the client, or some new thing is exposed (often literally). I’d expect the same is necessary for software consulting.

The bigger problem I don’t hear discussed in any of these threads, is that you can only estimate jobs you’ve done before. So, to do fixed bid profitably, it really requires that you are specializing in some set of deliverables. This, actually has some interesting benefits as well, one, is that you can charge a lot more over time, and you can become very efficient as well, which leads to a high profit per effort ratio.