It's not necessary to nip the clutch when downshifting because there's not much pressure in the tranny at this time. But it is better if you can give that clutch a little quick nip at the precise instinct of your downshifts as well.
Remember to keep one or two (one is better) fingers on that low end lever whenever you may even remotely need that clutch. On the start use two or three fingers but on the track for all those quick nips you'll be doing one finger is the way to go.
For all the Clutch, Throttle and Shifting techniques watch the free preview of my Volume 3 DVD #3 (Motocross Clutch, Throttle and Shifting Techniques DVD) at; http://wp.gsmxs.com/...clutch-control/