You'll need scales with one decimal place. I.e. scales that show "180 grn" are ok for general use. But to get arrows matched well, you'll need scales that show "180.2 grn". And make sure the zero doesn't drift...
Some digital scales come with a 100gm weight, so you can calibrate them.
However, absolute measurement accuracy is not very important as many scales will expose a difference of a grain or so between arrows. If you're trying to build a set of almost identical arrows, what's much more important is repeatability. I.e. it's not much good having scales that are specified as accurate to 0.1grn if when you measure one arrow several times the weight each time is shown as slightly different. When I was trying to build a set of arrows of identical weights I first measured the weights of the bare shafts, five times. Actual measured weights shown for the same arrow shaft were: 165.7, 165.6, 165.6, 165.4, 165.3, average 165.5grn. Then I did the same for the points, five times. A typical 100grn point was: 99.8, 99.5, 99.8, 99.8, 100, average 99.8grn. Then I tried to match points to shafts to end up with overall weights within a 0.7grn range i.e 0.2%.