I know what a great thing a mate is but when assembling a new stuff from random parts taken from a library - a combination of sketch picture capability of a part view and the capability of assembly view to manipulate objects freely in 3d space would be a great solution to quickly position elements on their target position and then bind them together with mates.
Unfortunately - there is nothing like "sketch picture" option in the assembly view. Importing a TIFF image onto the background distorts it while fitting into the place and it cannot be used for visual reference any more.
So girls and guys - what is your way (besides using mates) to quickly assemble a construction from library of parts using a visual references???