I am converting after-tax 401-k to Roth 401-k. Is converting a portion this tax year and the rest next tax year to spread out the tax burden allowable/smart?

I will have to pay income tax on the capital gains when I convert after-tax to Roth 401-k.  Is there a reason that I should or should not convert a portion this tax year and the rest next tax year to spread out the tax burden?