Seeking precision in drawing geometric fonts would be easier while adjusting to lower number of Grid Subdivisions. But this might limit several operations which if you do not pay attention to nuances, you may end up with distortions and blemishes.
It is expressed in moving handles; transformations and importing from external files.
Working on 2048 UpEm with a Grid spacing of 32 and 2 subdivisions while most of my font is build up with a consistent segment width of 160.
Started with importing from illustrator a square of 160 pt for testing into glyphs …the result came OK=160 (without any scaling) !!
But after importing a letter “E” where all three horizontal lines have the same 160 pt thickness; the result was after many pasting attempts into Glyphs: 160/160/176 !!
So I abandoned Illustrator and turned to do it in Glyphsapp.

A quick testing for a simple rectangle rotation results with minor distortion of which the rectangle corners snapped to the Grid and caused un-parallel distorted shape. {in cases with higher number of Grid subdivisions most of us do not notice such distortions}

Following up the previous discussions concerning Grid; I supposed GlyphsApp should have a quick menu switching of activating/deactivating “Snap to Grid” instead of redefining Grid parameters every-while in Preferences.

That could be done with a script. But why would you rotate the E?

If you have a grid and rotate stuff it will be distorted, there is no way around that. But make a test: Draw something, rotate it 10° nine time. It looks quite funny in between, but at the end it looks fine. So the rounding errors from transformation are minimal.

the “E” problem was not a rotation issue
I lost actual dimensions by importing previously drawn “E” from Illustrator that has 3 equal arms of 160pt.
the result was 2 arms of 160pt and the third arm = 176 !!

After moving to draw the whole font in Glyphsapp >>> noticed the Grid problem of distortion while rotating … etc

I join my voice to a long wish list that suggested a function of turning on/off snap to grid !

I tried again and again, but I cannot reproduce the copy/paste problem between AI and Glyphs.

Here is a grid switcher for you:

Update: Sorry, there was a mistake in the script, but I just fixed it. It should work now.