That's true - but there is another consideration at play: I'll always want to get enough wort to fill two 6 1/2 gallon carboys; if there is any wort left over, I'll pressure cook it in quart mason jars for use later as krausen, starter, etc.
I'll typically take an OG reading towards the end of the boil when my level starts to approach 15 gallons (I'm using leaf hops, so I have to account for what's going to be left in the boiler.) If the OG is too high, I may dilute with some water; if it's too low, I may boil for a while longer.