[crazygringo]

It depends on the index you're building.

If you're weighting purely by market cap then it's not very important, as you describe. (Accounting for newly issued shares is the only issue, but is often minor enough it can just be ignored.)

But if you're weighting by sector, for example, then it's hugely important, no? Indeed basically any weighting strategy except market cap requires rebalancing as a primary concern.