If you want your refractometer to read accurate at 1.000 then use pure water. If you want it to read accurate at 1.026 then I'd suggest using calibration fluid.
Without getting all technical, using a single point (either pure water or calibration fluid) to calibrate is only useful for a small range around that single point. To be accurate over a range of readings, you would need a two-point calibration, but the hobby grade refractometers we use don't have that option.
Some refractometers may end up being accurate over the entire range (1.000 to 1.026) and some might not. It all depends. Mine isn't, and is off by about 0.002 depending on what I use to calibrate it with. I know other folks with refractometers that measure perfect over the entire range. But you'll never know what one you have until you get some calibration fluid and try it out!