Sketching isn’t time consuming and is a really good way to put ideas in your head right on paper. After that, it’s always easier to actually design it on the computer, Sketching helps to evolve your imagination: once you understand it, you will always start from just white paper. What i do basically is i draw and i make a sketch on paper, then i take this drawn design, I scan it and I clean it up in Photoshop or with Corel PhotoPaint, then I trace it using the Live Trace feature in Adobe Illustrator or by Power Trace tool in CorelDraw.
I do a lot of work with Illustrator and with CorelDraw to create new designs based on the sketch drawing and the vector art it produces can be used in numerous ways and is easily customized, see this (Clenched Fists) designs collection:
Another Example of (Hands Victory) Design:
Another Example is a Celtic Tree logo design, you can see the sketch drawing and the final design:
Bull Logo Design: