The problem is that you can’t just “fire” bad customers because you don’t want to face a lawsuit. They paid you to launch the results, and you have to be willing to do what it takes to finish development, get it launched and then go and fix the bugs. You can re-use most of that in your SAAS later.
If your customer is a business then their needs and desires are probably unique to a degree. A high-maintenance customer will mak a ton of requests that specialize your product to them only, and be little value elsewhere. If you don't know yet where that line is, that's fine. You can learn as more customers come along. But I certainly wouldn't call it being paid to build what you should be building anyway. More like being paid to operate as an independent contractor while you learn the industry.
One off projects, yep you better do everything what client says and don't forget to chase after them to pay up even if everything is done to spec.