With the bike on the mainstand, make sure the front wheel is off the ground. The wheel should be able to go from side to side without feeling any resistance from the bearings. Tighten the head bearings just firm enough to feel that there is no play in the bearings after swinging it from side to side max. If it feels tight at all it is too tight.
If you have a tendancy to centralise, or feels at all clunky,, the bearings both need replacement, as the will be gouged. it is a good idea to swing the head from full lock to full lock at least once a month no matter if ridden or not, as this will assist lubrication.