You are in luck, in a way. Apex has a combined water treatment plant with Cary (I live in Raleigh, NC BTW), and typically the water is quite soft, so there's little in the way of dissolved compounds in it. This is assuming that you are on the municipal system, not a well.
Either way, you shouldn't use tap water as evaporation make-up. There are two potential problems: accumulation of ionic substances (sodium, potassium, manganese, magnesium, iron, etc...) and if you're on the municipal system, chloramine.
If you're on the municipal system, accumulation of ionic substances will be much slower than if you're on a well because the municipal systems around here supply water that is quite soft (low total dissolved solids). However, you're still going to wind up with an incorrect balance of salts in your tank water over time, since the dissolved ionic substances in tapwater don't match the ratios in seawater.
Chloramine is deadly to any aquarium inhabitants (freshwater or saltwater). And while you can neutralize the immediately toxic effects of chloramine with Amquel or something similar, the nitrogenous compounds that remain after neutralization are still fertilizers, which isn't good for a marine tank.
Bottom line - either purchase your water from The Fish Room in Cary (only local store I know of that's close to you that sells RODI), or stretch your budget and buy a basic RODI system that is designed to deal with chloramines.
By the way - there's an even better reef shop in Clayton called ReefKeepers. The owner really knows what he's doing, and has some spectacular display tanks, but it'll be a little bit of a drive for you from Apex. Full disclosure: I have no connection to either Reefkeepers or The Fish Room other than being a frequent customer.