From your description I presume that you have a threadless steerer setup. If so the only purpose for the cap bolt is for setting the headset preload (and holding the cap in place of course). After you are done, the adjustment is held by tightening the stem clamp bolts. For all practical purposes you could remove the bolt (and cap for that matter) altogether with no ill effect. So there is no point to being concerned about the tightness of that bolt.