Why prepare?

First of all, I don't generally recommend it any earlier than week 12 or 13 (depends on your league's habits).

But the main reason for planning D/STs specifically is the need for security. D/ST projections fluctuate week-to-week significantly more than any other position, because they depend so strongly on the opposing offense. Just looking at my model, the standard deviations of most positions' predicted scores are usually 1 point-- and closer to 2 points only for QBs and top RB1s. In contrast, most D/ST projections fluctuate by at least 2 points and up to 4 points. This means it is unlikely that a single D/ST will have schedule+skill to consistently award you top points, for more than 2-4 consecutive weeks.

That's the reason for streaming, and that's the reason for looking ahead to the wins that matter most.

Why week 12?

The right time for you to "stash" depends on 3 factors: (1) the timing of your leaguemates, (2) how shallow your bench is or how shallow you can tolerate it, (3) how many weeks of high scoring can be "secured" in advance (often 5 weeks, maximum 7). With 3 weeks before playoffs, it is now possible to lock in with some combinations.