Weather. It is always the weather (and it sounds like you might already know this). It is great to plan far in advance, but if the weather is bad on the upper mountain (i.e. ANY precip and wind) it is best to have a solid plan B (Stuart Range?) and save Rainier for another trip. It is no fun up there in a storm, and often dangerous. Early May is often unsettled. Or, it might be just fine. Good luck!