There is a strange behaviour with boolean operations:

For example, I draw two squares with 10x10mm and no outline and place them next to each other by removing the overlap.
Now, I want to combine them because I need a single object as one area.

If I mark the two squares, the dimensions are correctly shown as 20x10mm.
If I union them (Ctrl- +), the dimensions of the resulting object are 19.994x10.001mm
If I combine the original squares (Ctrl-K), the dimensions are still 20x10mm
If I union them after combining, the dimensions are 20.002x10.001mm

In several another posts I read that this might happen.
But how to avoid it?
Increasing the numeric precision e.g. to 16 (instead of default 8) in the Inkscape Preferences did not help.
Thanks for hints.

Cannot replicate the issue here... :-\

I snap cusp nodes to cusp nodes.

OK, I had Inkscape 0.48.5 on Win7...
Now, I installed 0.91pre2_64bit r13532
and there I do not see this effect anymore...