First of all, you should not necessarily tighten the mainsheet. Centering the traveler will not center the mainsail but will actually leave the boom to slightly to leeward. This slightly eased mainsail position will help with acceleration out of the tack and will reduce the likelihood of a knockdown if you turn too far through the turn.
As to the placing the traveler windward or leeward per the wind speed, in light air you want more twist in your sails up wind becase the air speed is faster higher up and so the sail up high is sailing in different wind than the sail down low. In lighter winds the traveller should be well to windward and the boom should be on the centerline of the boat.
In heavier air the sail should be bladed out and so the traveler should be carried well to leeward and a lot of mainsheet, outhaul and halyard tension to flatten the sail.
I suggest that you might look for a book with a chapter or two on sail trim. I am not putting you down here (we all had to start somewhere) but these are hard concepts to explain over the internet.