If you pitched last night, then don't bother, you won't know what your SG really was. If you're worried about your starter changing the SG, as long as the volume of the starter isn't that big, or it isn't a really big beer, it shouldn't make much of a difference.
Ah, a slightly different question. Your initial phrasing made it sound like you were going to check it this morning. Pitching the yeast, then taking the gravity won't change the reading, unless you pitched 2-3 quarts of starter.