Yes, it's generally true. Sometimes when you buy a package through one company for both the domain and the hosting plan, you find in the fine print that they have retained the actual ownership of the domain name. Thus, unless you want to move to a different domain name and lose everything you've built up in the way of membership or customer base, and search engine results, you are trapped at that host. With separate registration and hosting, you have the opportunity to leave your host and go to another one, with no loss of the domain and all its value. You also have the opportunity to transfer the domain registration to another registrar, if you are unhappy with your current one, with no loss of your site. The only downside is that you might pay a little more to have a separate domain registration and hosting plans.

While LunarPages does offer combined registration and hosting packages, I have not heard any complaints about their retaining ownership of the domains (I registered mine at another registrar, so I have no first-hand experience dealing with with LP). Supposedly, the registration is with TuCows and is separate and fully owned by you. As always, read the fine print.