Yes, that is true, to be percise, you need the calibration fluid and to go further, be in a temp controlled room at the temp that the instruction say to be at. To add to this, pressure altitude (barometric pressure)will change the SG
of liquids as well. All well within the margins we use in aquaria. A .001 +/- is close enough for most that the RO
versus calibration fluid will generate.
Most meters are self temp correcting to temp but not to pressure altitude. Want to test it, watch the weather and as a storm approches use the calibration fluid and the once the storm is present, test again and it will be different. Probably more drastic that the differene between RO
and calibration fluid.
In my feild, I us SG
on a regular basis and have to adjust for temp and pressure.
But, I digress, if you want to do it totally right, use the calibration fluid, and you should test your refractometer rior to every use. It is probably best to use the fluid to make sure your meter doesn't go defunct.