I batch sparge, and will take a sample from the first runnings, and set it in the freezer to cool while I continue. It usually turns out to be about 1.080. I'll take another sample from the second runnings (1.035 or so). I aim to collect about 7 gallons, so I just do the math to figure out how many gravity points I got from the mash. All this just lets me know if I'm in the ballpark. I take a "real" sample just prior to pitching yeast (this one I don't put back, of course).
"Anything worth doing, is worth doing slowly." ~~ Mae West