We use a harness, to good effect usually. This year, our ewe put out both lambs and her lamb-bed early through it - so although it was a sad outcome, as we lost her, I have stopped worrying that lambing can't start/happen with a harness on.
So, if you are around all week, I would leave the harness on, keep her inbye and watch her closely and take it off when you see the water bag appear.