Hi Silvano - yes, this is a known bug with, as far as I know, no good solution yet. It is more obvious with more extreme scale factors, but it is always there I think. I’ll see what I can find out about getting this fixed.
Thanks for putting it on the bug tracker. In case you have a priority list, could you move this one up. It is extremely annoying and makes Rhino almost not useable in drawings with many dimension lines such as architectural drawings.
For others having this issue too, I am using the following work around. I lock my dim layers when working on other geometry and when I have to edit dims I switch all of them into point edit mode (PointsOn).