You do not tie off the thread, but you carry it across the back of the work as long as the distance us under an inch or so. This is one reason why you work from the lightest colors to the darkest. If a dark thread is carried across an unstitched light area, it will show through. But if the area is already stitched it won't.
You might be interested in either my free beginning needlepoint class or my beginning shading class that starts in mid-October. Both are email classes and the beginning one is free.
Please email me at email@example.com for more information.
Depends how neat you are and how bothered you are about it if it looks like spaghetti :)