If your gears have a lot of time on them and have mated with a good pattern,do not attempt to take up any backlash from wear.You will put the gears on the unworn edges of the gear faces and depending on the load can cause noise or even breakage.
Backlash does not cause problems unless the gears are worn out,or it is a reversing drive and the noise is objectionable.
Spiral bevels,hypoid and other forms besides straight tooth gears,once set to the correct pattern should be run with the original settings until they wear out.Attempting to move them closer to take out backlash moves them from their mated/seated position and can lead to heel/toe/flank misalignment and excessive wear and breakage.That being said your drive may be very lightly loaded and not worn much,as already mentioned the marking from your bluing test will show how they are meshing if you know what you are looking at.
After you assemble them hold one gear shaft and turn the other both ways,a proper set up you will not feel the gear teeth engage,it will be smooth.
Another thing if your gears have depth and b/l markings on them,they were sold as a lapped , matched set and that is what they should be set at.