getting the majority of your media to match your sequence settings is important in FCP7
so unless you have added a lot of fx, then the majority of your timeline should not need rendering.
if it does then you’re doing something wrong, and you have to stop and figure out what that is
if you HAVE a lot of fx, or even if you haven’t,
its always smart to FULLY RENDER your entire timeline.
this means going to the Sequence Menu > Render All, and ticking “Full”,
and hitting Option R when you want to render
once you’ve fully rendered, the export will be much faster,
and of course when you have to change one little part, you just re-render that bit,
and are ready for another fast-as-possible render.
nick