Oy. Don’t follow advice from people on the internet that don’t know what the F they’re talking about.
It does not matter whether you roll the 401K into an existing or a brand new IRA account. The computation for recovering non-deductible contributions is exactly the same. Having a separate IRA for the rollover won’t change anything.
There could be difference with the Roth 401k, but only having to do with the 5 year rule, and the order in which non-qualifying distributions are treated. (Non-qualifying distributions are those made before age 59 1/2 and/or less than 5 years after the Roth was created.) If all accounts were begun (Roth 401K and regular contributory Roth) more than 5 years before distributions and all distributions are after age 59 1/2, then it won’t make any difference.
