How clean is clean before restaining?

I have a cedar deck that's 6-7 years old.  I've since stripped and sanded the original contractor's stain and restained with TWP101 Cedartone.  Cleaned and restrained that once two years ago. Didn't do this last year, so the deck got pretty weathered.

A few days ago I used RestoreADeck to clean and brighten.  Left the deck half stained half bare wood, and as you can see the bare wood is light/med gray.  Can I restain over this mottled pattern, or do I need to start over:  strip/pwr wash/restain, the whole nine yards?  See photo.

Also, for future maintenance, do I need to clean/restain each year?  We're in SE Washington state and receive a fair amount of morning sun in the summer.

Thanks.