Without re-balancing, your portfolio won't stay 60/40. If your asset allocation falls too far out of your accepted parameters, you will no longer be following your investing plan.
Basically, you're choosing an AA with a comfortable level of risk/reward, and you rebalance to keep your portfolio within that level of risk. It's not market timing, it's the opposite of that. You have a predetermined plan (asset allocation) and you rebalance your portfolio periodically to assure that you're following that plan. You don't even need to know how the market is doing, you just need to pay attention to your own portfolio's percentages.