This would be a really easy way for the OP to figure out if temperature matters for his hydrometer.
Pick a time after the engine has run to hot and hot coolant has gone into the reservoir. Park and take some out of the reservoir and test a portion. Then continue testing a portion every X number of minutes until it cools to ambient temperature. If there is a temperature effect over that range, it will show up.
Science! Gotta love it!