What do you think the timecode is...? It's not something that is magically prerendered into your video. It's just like any effect you put into your video.
Zippy,
a good idea if your'e working on a big project or one with lots happening is to render out bits as you go.
As far as I can see there's no down side to this in terms of quality if you use a bit of intelligence in how you do it. This way if you want to do something like add timecode then sure it's going to mean another render pass BUT only that FX has to be rendered in and can be much faster.
I always do this if I've done things like alter velocity etc, its a hard enough thing to get my brain around as it is, rendering it out as a new clip saves me a lot of confusion.