It means that anyone mindful of how wood behaves/reacts is cautious.

Where a tree grows, say a windy edge of a wooded area, or in the middle of a field, will place internal stresses into the wood fibres that are locked into place as it grows. These stresses can cause movement in the wood as a consequence of cutting/planing/humidity fluctuations and so on. No two bits of wood are the same. 

Before this bit of a tree has lacquer and strings slapped on it, we want to be sure it doesn't think you're William Tell/Robin Hood and only plays "Meet me on the Corner "