The sneaky part of asset allocation - when you rebalance - you automatically are taking market conditions into account - by rebalancing into the 'better buy' - relative to your other asset classes.
Of course - if you lose discipline, get greedy, or emotional - welllllll
? - heh, heh, heh.
Then there are old turds(no names please!) - who just own Vanguard Lifestrategy(Target Series is also good, even - psst Wellesley, or at a stretch - STAR if you are very young).
Those trusty computers at Vanguard just rebalance their little hearts out - even while you sleep, go fishing, watch football or whatever.