I don't think down tuning is essential. a good riff/song should sound good in any key, if it doesn't then it ain't that good.
I kind of disagree with this, when we play new riffs it matters a lot where on the fretboard they're being played.
And normally there's a proper place for every riff to be found, a suitable sound.
My guitar player tends to start everything in C but then it all sounds too boring.
I agree that a good riff sounds good anywhere but the character and mood changes quite drastically when played on different positions.
Also it's of huge importance to find a sweet spot for the vocalist(that would be me), I hardly imagine our music uptuned in D or E and having to sing over it.
I like C and will likely play in it always when playing "stoner", well maybe I could go up to D sometimes.
Thing is I suspect my ThunderBird would sound better in D.