How do you manage the bid-ask spread not eating into gains when rebalancing? And what portion of the an index fund's 0.1% expense ratio "pays" for that? Also,

How do you manage the bid-ask spread not eating into gains when rebalancing? And what portion of the an index fund's 0.1% expense ratio "pays" for that? Also, how do you position yourself compared to Fidelity's Custom Investing product?