in xaml, is it bad practice to use a frame, inside a window to display pages?

I was just wondering what the common practice is, to have pages (I dont mean pages as in XAML pages, I just mean...different screens) which can be navigated between?

Right now I am using just a bunch of pages, and using the NavigationService class in C# to navigate between my pages. But I just discovered Frames.

Is it better to have a frame inside a window:

<window>
    <frame source="page.xaml" />
</window>

or just do it with all the different pages and the NavigationServices? Or should I be using a window inside the frame?

What do you do??

Thanks!