Shroud tension is set with the jib halyard. Some people like loose rigs, some like tight. I've always had a "semi-loose" rig, which allows the mast to rake a little forward going downwind.
If the rig's too tight, then the mast will want to rotate preferentially to one side, typically the side with the jib halyard cleat. No matter how tight you think you have the rig, though, the leeward shroud will always be slack when sailing upwind.
If the rig is too loose, you run the risk of the mast coming out of the step in a capsize - a very bad thing.