You probably can't get it tight enough with deadeyes to damage anything. Even if you lead the fall of the lanyard to a handy-billy or winch
, it's not nearly the same as reefing on a turnbuckle. Soooo, as tight as you can get them.
Is the schooner gaff rigged? if so, less tension is required than with a tall marconi rig, though you should still strive for enough tension that the lee shrouds don't flop around.