By waiting to do the conversions, the balance in your after-tax balance may increase in value. These earnings would be taxable income at the time of conversion. If (for example) you're in the 25% tax bracket, $100 of gains means $25 in taxes.

Suppose you invest $2,000 in your after-tax 401(k) once per month and you expect your investments to increase by 1% monthly (as a simple, but optimistic example).

If you do the conversion every month immediately after contributing, you'll have no gains to pay tax on, but you'll pay 12 * $25 = $300 in fees over the course of the year.

If you do the conversion once per year, you'll only pay a single $25 fee, but you'll also have roughly $1,300 of gains to pay tax on. This adds up to $325 in extra taxes in the 25% bracket. Total cost: $350.

If you do the conversion twice per year, you'll pay $50 in fees, and will have about $600 in accumulated gains to worry about ($150 in taxes in the 25% bracket). Total cost: $200.

See where I'm going with this? Plug in your own tax rate and growth assumptions to see what ends up being optimal.