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I've also used some big clamps to keep some of the bendier timbers in line
Be careful when you take them off! It's unlikely but they could spring back with force.
While you are doing no harm by doing this it won't have much effect in the overall straightness of the timber. Generally when you use bendy timber you arrange it so that the curves work for you, for example when you are joisting out a floor you place the timber so they are "crown up" (so the top of the curve is on the top side of the joist). This way gravity will pull the curve back relatively straight and you won't end up with a big dip in the floor.
When you're storing / seasoning timber each row should have smaller timbers placed across them at regular intervals that the next row lays on these smaller timbers are called stickers (at least that's what I was taught there are about a thousand different names for them).