Does Structural Lumber Warp?
High moisture content (wet use applications) plays a role in lumber's strength, but not in this case. Yes, structurally graded lumber will absolutely warp/twist/shrink/check. When selecting lumber it is best to look for KD Doug fir lumber that has been kiln-dried to a moisture content of 19% or less. That'll help it remain dimensionally stable. Honestly, the "green" undried stuff will work fine for building a shed. Doug fir is pretty stable in general compared to pine lumber, especially southern pine.
For an 8x12 shed the structural considerations are pretty minimal.