Phosphate is a commonly used precipitant, ammonium sulfate is another commonly used precipitant.
Sulfate is a phosphate mimic.
Clearly, the use of phosphate for the crystallization of a protein that binds phosphate moieties, does not follow rule 2. However, reverse screening does not prohibit the use of phosphate or phosphate mimics in the crystallization of proteins that bind phosphate moieties. It suggests that we should be aware that phosphate is affecting the state of the protein at the same time that it may be inducing supersaturation. We may be loosing control of the crystallization. Using high phosphate concentrations will ensure that the phosphate sites are fully saturated.