I think until you are close to going from all fish to reef you are fine with regular salt. Once you get ready to add coral start doing water changes with reef salt. Reef salt won't hurt to use for fish only.
I'd be thinking, at this point, more about lighting. Planning and research is key to a successful salt water tank. Lighting is expensive but essential to coral. You need better lighting with the correct wavelengths of light for coral. The kind of coral you want will dictate how intense your lights need to be. You may save a couple bucks on regular vs reef salt but reef lighting will cost you quite a bit more than what you can use for fish only you don't want to buy lights twice.