I checked my bike for play yesterday and found some in the headset when rocking while slamming the front brake. I loosened the stem bolts and top cap then tightened them again. The play in the headset is gone. I just checked the play in the spacers for my own curiosity. I see that that the spacers below the stem have no play, cannot even rotate. The spacer above the stem can rotate quite freely but has no sliding freedom. Does it need more tightening in the top cap? By the way, I do not have a torque wrench, I am tightening by feel and check.
When the stem is clamped in place, it's enough to keep the headset in place.
The top cap is used only for adjusting headset bearing preload and keeping water and dirt out of steerer tube, so in principle you could even remove it after tightening stem bolts and save a few grams of weight. You could as well tighten it without interfering with headset adjustment if you like.