Well, I assume you are doing extract partial boil. In this case your OG will likely be spot on because you know exactly how much malt extract you are putting in and it has a set amount of fermentables. Your OG will be way high before you top up with water and unreliable after topping up as your wort will not be properly mixed in most cases.
When doing extract, you are much better off using a beer calculator like beersmith to calculate OG and then using your hydrometer to check your final gravity to determine when fermentation is finished. It is much more important to check OG when doing all grain brewing as it will tell you your brewhouse efficiency, which you can then use to dial in your recipes to start hitting desired OGs.