When I started playing with water chemistry I bought myself a scale with 0.1 gram precision. It's perfect for measuring any small addition, such as potassium metabisulfite, yeast nutrient and hops. With hops I've found that the weight on the package might not be what you actually get. So it's good to weigh out your hops!

My homebrew store carries pre-packaged hops from Hopunion in 1oz and 1lb sizes. I have a lot of recipes that call for an even ounce of hops, so I normally just open up the hop package and dump in the whole thing. But over the past few batches I've weighed out the hops and found that some 1oz bags of hops contain an extra 3-4 grams of hops. For a late hop addition that's no big deal. But if your bittering addition is off by 3-4 grams you could be adding 10 IBU to your beer (more or less, it depends on the hops and the recipe, obviously).

So pick up a scale designed for 0.1 or 0.01 gram increments and measure those hops!