I do remember reaing (just don't have the sourse) that hydrometer temperature corrections are not linear and the further you get from the calibration temp, the more error is introuced.
+1 to several comments to cool the sample. It's quick, cheap, easy and more accurate. Never thought of the cocktail shaker, but I like it. I'll typically set my pyrex measuring cup in a pan with cold water/ice and swirl it around. That 3/4 cup or so of wort cools pretty fast.