1. You NEVER should scratch build a system for a company, even a small company. Doing so exposes you to support headaches and additional risk. And by "you," I'm talking about guys building PCs for people on the side. If you own your own computer shop you can do it, but it still isn't a good idea IMO. Consulting services are where the money is at, not slinging commodity items. I have a friend who owns a small "consulting" business and he gets caught up supporting PCs for small businesses instead of looking at the bigger picture and selling higher-margin, more profitable services. Drives me nuts!
2. I would extend the above and say that even building PCs for family and friends is something you should be careful with. I spec out and help my brothers build their PCs, but only because I know they're moderately savvy and won't whine and complain to me with every little problem. On the other hand, I would never build a computer for my mother-in-law. They had a Dell and wanted to upgrade and I told them to get a Dell again. I've helped them with a couple of issues but no way would I want to be on the hook for full-time support.
3. I personally build my own PCs and as I mentioned earlier, still advise my brothers to build their own. Why? I still find it an enjoyable hobby and like to know every single part that goes into a PC. I also build my own servers as well.