Not necessarily. You can use cm as default units and draw your stuff watching the sizes in cm in the x/y/w/h controls. Then just open Export and set 300 dpi there. No need to manually multiply numbers.
I dont know, if it is only in svn, but: If I set File->document properties and set the default unit to mm (not pixel). Save the document, and reopen it, the default unit will be revert to pixel.
The bigger problem, that all the unit saved in .svg file are in pixel. So there are rounding error everywhere. If I want to create some technical drawing (in mm), and have some exact line (say 40mm long line), as soon as I quite inkscape and reopen that file, my line will not be exact 40mm long.
Is it a know problem?
Ps: I really love the new dockable dialogs. Its so easy to work with inkscape with this feature added. Thanks!
Best regards, Khiraly