pH will vary with temperature, dissolved CO2, and ongoing chemical reactions (especially metabolism of any bugs). I'd freeze the samples to slow any chemical reactions, enzymatic activity, and any yeast/bacteria that get in it. But cooling will change pH a little and allow more dissolved CO2. Ultimately it depends on how buffered your water it. High hardness will be more resistant to pH change - both during mash and during storage/transport to pH meter.
Random Fact: Human Immunodeficiency Virus particles have approximately the same density of 1.170 wort. I know because I float them on a sucrose gradient in the lab to purify them for research.
Drinking: Belgian strong golden, Northern Brown, Porter