If I'm pitching into 5 gallons with a starter, the starter is generally going to be somewhere around a quart after pouring the clear liquid off. So that's 5% of the total volume. It will increase the gravity, but not much, and a lot of the increase comes from stuff I'm not going to keg. If what you're pitching is mostly yeast, it's going to settle in the fermenter eventually, and that will greatly reduce its influence on gravity readings, so is there any point in measuring after pitching? I don't think anyone shakes a fermenter to put yeast and crud into suspension to get a truly accurate reading.
This is one of those things I wish I had never been confronted with, because I make perfectly good beer measuring my OG after the boil and before the pitch. I don't use starters unless I have to these days, so I guess the whole question is not very important to me.
There is a ton of bad math and approximation in brewing, because no matter how much we try to turn it into engineering and science, it's still an art. If you get good results, don't drive yourself crazy trying to be rigorous.