If you are adding "top off" water to the fermenter, make sure that it is well mixed into the wort before you draw the sample for the hydrometer reading.
If you follow an extract recipe (or kit directions) correctly, there's almost zero chance that your OG will be substantially different than what the recipe says. In other words, it's not really necessary to take a hydrometer reading at all. But it might help you feel confident that you did it right, if this is your first batch (or close to it).
Bottle conditioning: Robust Porter
Drinking: Saison Dupont clone, tripel
Coming soon: Columbus APA, Rich Red ale