Why is snapping to the Grid in Blender 2.8 not (always) reliable for precision for centimeters? I've modeled objects that I wanted to adjust some vertices for so that way the edges would have an even measurement. (I use the Measure it tool to determine the distance between vertices.) However, I found that when I would use Absolute Grid snap to snap the vertices to the grid it wouldn't always give me a precise measurement despite the fact that I would have set the units I wanted (my units are set to centimeters). 
For some reason the measurements work much better with meters, but for centimeters specifically, it seems like there's a slight margin of error. 
I'll show what I mean.
The first 2 images are the same picture. One is just zoomed in a little more. As you can see both decimal points are on the squares of the grid and I made sure it was snapped to them.For some reason though the units aren't exact. It seemed a lot more exact with Meter measurements, but no so much with centimeter measurements. I then went back in and checked with the measure tool and snapped it to the 2 vertices shown on the picture and got similar results. I don't know why it's like this. Perhaps it's just something about Blender measuring the centimeters after you've transformed an object a certain amount of times. That's why I was wondering if there was a solution or work around to this.
A few things to note about 'Snap to Increment'


In Object Mode, it snaps the mean position of the pivot points of selected object(s), in Edit Mode, the mean position of directly or implicitly selected vertices.
It snaps to the grid increments you can see. (If you're zoomed in, the increments may be subdivisions of the increments when zoomed out)
If Absolute Grid Alignment is switched off, you are constrained to move elements by units of the smallest increment you can see. If switched on, you are constrained to move elements to the nearest grid point. (In the first case, elements won't necessarily wind up on a grid point)
Whatever your view, the snap is to an orthogonal projection down onto the grid you can see. If you're viewing at an angle, or in perspective, the snapped element won't necessarily appear to coincide with the grid you can see, until you adopt an orthogonal view straight down onto it.
If using in an orthogonal view, it will also snap in the dimension you can't see. (So for example, you're looking straight down Z, snapping in XY, starting off-grid in Z, there may be some unwanted movement to the nearest Z grid-point, unless you constrain the movement. (GShiftZ)


A few of these things might give you the impression the increment snapping 'isn't reliable', and can make it counter-intuitive to begin with . But it works as expected. 

EDIT: This answer doesn't really answer the question, since it has become one about precision.. but I'll leave it up..