I was in Death Valley in late January and visited the Stovepipe Wells dunes several times, both early morning and late afternoon. There were few people actually on the dunes when I was there, but there were footprints pretty much everywhere. And the famous nightly sand storms weren't happening to clean the dunes either. But I found that if you went far enough in, near the big central dunes, there was much more clean sand. Here are some samples I took:
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ent]
Of course, judicious use of the clone tool afterward helped.
I do recommend January for DV, as the crowds haven't appeared yet.