That should not be too difficult to be done and it will be very helpful! :)
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It would be great, indeed.
For now, I personally use the WebDeveloper Toolbar - plugin (available for both FF and Chrome). That contains a ruler-function. That is sub-optimal, as you need to stop drawing and use this to draw the lines on screen, but it /does/ help to get things lined up perfectly…
There might even be better plugins to use, but I allready have this one, so…
regards,
Paul
Thanks for the advice! 
Another useful suggestion would be Motion Guides I think :)