Earcandy - there are 2 different types of freeze.
You can freeze a track, or you can freeze a synth.
I personally never freeze a track, as it's so much easier to freeze the synth.
This doesn't freeze your Fx, so you can mix as you go, add whatever processing you want, but if your system starts to suffer from dropouts or slowdows or you simply run out of CPU horsepower, you can help by freezing the synth to remove them from memory.
If you want to edit your Midi later at a later stage, simply unfreeze the synth, make your change & re-freeze, and you're back in Audio land.
BUT. If you have a reasonably modern machine you might find you never need to freeze anything and you can run your exports directly out of the project as a wave file