The best way to alter materials like posts, studs, rafters etc and have the takeoff remain accurate is to "edit" the group and use the Push Pull tool to lengthen and shorten the object. Depending on the object you may have to "drill down" with multiple right click Edit Group (or component) moves to get to individual studs, rafters etc. This works like a charm and takeoff quantities are correctly added or subtracted.
As an aside, you can copy studs, rafters and the like out of groups, paste them individually and use normal Sketchup tools to later, cut, shorten, lengthen them etc and they will be accounted for in your takeoff. I do this regularly and over time have built quite a library of objects that I can cut and paste between projects and have them correctly accounted for in my takeoffs.
Scale tool works on materials like insulation, building wraps etc as with those you are directly scaling the "square area" of one plane.