My deep 6, on the package, said it should be soaked the first time for 24 hours, and again if it dries out. Might as well just leave it constantly submerged then. I've heard various others state the same. And my girlfriend's dad makes *ahem* moonshine, and he has to soak his hydrometer at all times or else he might make cyanide by accident lol. It's a swing needle hydrometer meant for wine/alcohol making.
from my own experiences, mine would fluctuate in reading according to my refractometer unless it was soaked recently.
I mean calibrating by finding the variance of your meter using a refractometer. here's a good article explaining hydrometers, their good accuracy, and how to get them accurate. A better term than "calibration" like i've used, would be the author's term "correction factor." It's about finding the correction factor for your own hydrometer.
http://www.advancedaquarist.com/2006/9/lines
sorry about the confusion over "calibration" lol. I meant "correction factor"