Thank you everyone for your responses! They were tremendously helpful!
Just to bottom line things, I think what you all have essentially said is if you are using KNO3 and KH2PO4 to maintain your nitrate and phosphate levels per the EI method, then your potassium levels are probably just fine. The exception to this when you have a heavy bioload or other source of nitrates and/or phosphates, hence you need to back off of the KNO3 and KH2PO4 so you don't overload your tank with nitrates and/or phosphates, and therefore you need another source of potassium. Does that sum it up?
Is it correct to assume that if I include the "optional" K2SO4 into macronutrient liquid (per the link that Maryland Guppy provided) that the extra potassium and sulfate are not going to harm anything?