There's no reason to let it grow in the Traditional IRA before converting. You're just throwing away the basis.
Scenario 1: deposit $6,000 in traditional IRA. Wait until it's worth $7,000, the convert. You have $7,000 in your Roth, no taxes owed, and no basis in your traditional.
Scenario 2: deposit $6,000 in traditional IRA. Convert immediately. Wait until it's worth $7,000. You have $7,000 in your Roth, no taxes owed, and $1,000 basis in your traditional.
Why would you ever choose 1 over 2?