George, you can definitely do that (start your SS at 129) but doing it to avoid future changes?
I am in a similar situation, wanting that DMX universe out of the hub, so I am intentionally avoiding the use any of the first 512 channels for Smart String use. That way, if I do need to add another Express or 3-channel controller or MR16, etc. on that universe, I have plenty of spare channels.
Smart String channels have "high numbers" anyway, so by avoiding 1 thru 512, I'll always know what is DMX and what is SS. You'll still have 7x512 channels for PixelNet purposes. (3584)