pH probes should be kept moist. there is 'storage' solution for this which is the best. then distilled water, then 7.0 calibration solution. this what my local shop has told me. like freezerboy said, any other water beside distilled will shorten the life of the probe
My Oakton EcoTestr pH2 does not state that it should be stored in solution.
I have been using mine for nearing 18 months now, never stored in solution. It needed recalibration once after replacing the batteries (also needed only once so far, after close to a year).