[EricH]

It depends on time available and accuracy needed. It also depends on whether nearby items need CAD.

If I'm low on time, and I don't have a CAD, I'll grab paper and pencil/pen and do a freehand or ruler-assisted drawing. This is also good for a quick concept model. I do pretty well at freehand perspective drawings, though.

I'll often turn the items drawn up into CAD models, too.

A good perspective drawing course and some practice can really help you make crisp, clear drawings. I've been known to make two quick sketches in half an hour to an hour that wound up in a PDR slideshow after conversion to electronic form. One of them had to show internal structure as well as outside skin.

The time (and maybe money) investment in learning to draw with pencil and paper is well worth it. You might not produce full-on shop drawings, but you'll be pretty close.