I would think a series of additional test solutions would be in order... AFAIK, Crystal Light <shudder> has Nutra Sweet, or aspartame, as sweetnener. Aspartame (a methylated tripeptide) is markedly different from Splenda (which is a blend of various carbohydrates) and Truvia (stevia based, so derived from a plant glycoside)...I wouldn't take one measurement from an aspartame based solution and extrapolate to these vastly different sweeteners.
To confirm your reply though, I would think that adding any one of these products would potentially affect your refractometer reading (and perhaps unpredictably, and not reliably correlating to readings taken by hydrometer), so yes, I'd get a reading before adding (and after, of course, just for comparison and curiosity's sake...)
The bigger question is why are you feeling it necessary to backsweeten with these? If you're just looking for something unfermentable, I've had good success with lactose (understanding dietary issues notwithstanding).
My main intent is to add levels of sweetness that will not be diminished from yeast activity, like a hard cherry-limeade which I have fermenting now. Since I have no means of force carbonation, I am trying find/found a work around. The problem with back sweetening it is the fact that the bulking agent, which its name is eluding me, is fermentable. Adding it it at bottling time turned it into almost soda levels of carbonation, nto what I was aiming for, but quite tastie none the less. I didn't take any reading because I had my doubts about getting valid readings/ samples.