I think the trouble is Photoshop having to convert from physical units (Inches, metric, points) to pixels. In the Canvas Size dialog, if you enter your size in centimeters, you can change the measurement to pixels, and it will show how many pixels that would be. Since you want the same size on all sides, it has to be an even number, because there are no fractional pixels.
If the value is odd, you would have to change it, and decide to go up or down. If you do this often, it could be scripted.
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