It wouldn't effect your weight loss either way, but the Ideal Protein diet is not just for losing weight. It's also to retrain our bodies to most efficiently process the energy we consume. Different sweeteners can effect our bodies differently, even if it doesn't effect our weightloss, from the way our digestive tract reacts to it, to changing our bodies' pH levels (which should be a little more basic during the early phases of IP).
Just avoid using any other sweeteners besides Stevia and Splenda.