Many thanks for the pointers - I figured I should just go RTFM rather than whining on lwn about stuff like this, thank you.
I don't mean to go on, and I'll stop after this, but one other example of why I prefer not to use inkscape for CAD work. This is with inkscape 0.48, so may have been fixed in 0.49.
1. Start inkscape.
2. Set document units to mm (I can't find where to set this as default in preferences, but I'm willing to bet it's there).
3. Quickly create a box, any where, any size - doesn't matter.
4. Select and Path -> Object to Path
5. Now set the X, Y, then Width and Height to nice round numbers. I set X to 30, Y to 140, then Width to 70 (and watch! - X now changes to 29.974, and Width to 70.052), then set Height to 70 (Y now changes to 139.994 and Height changes to 70.012).
This is repeatable, in so far as the X and Width change when you set Width, and the Y and Height change when you set Height. How they change is less predictable. In case it makes any difference, I have now set to Geometric bounding box. Yes, I know I really should go check 0.49 and then file a bug if it still persists.
If people are successfully using Inkscape for proper drafting work then this really must be me somehow - I can't believe anyone seriously using Inkscape for drafting would be willing to cope with this sort of problem.