In the US, Roth contributions are made with "after-tax dollars", while 401K contributions are generally made with "pre-tax dollars", so if you rollover the 401K into a Roth, the IRS will see the rollover (aka "conversion") as a bump in your income and ask you to pay tax on the rollover amount, with the remainder/net going to the Roth IRA.
I still think a Roth conversion is a good idea for self-directed investments, but folks should be aware of the incoming tax bill.
[1] https://www.investopedia.com/articles/retirement/09/roth-401...
My tax rate will be higher while I'm working and earning a salary than they will when I'm retired.
Also, by keeping the base amount higher (pre tax), won't I have more growth than compared to the after-tax amount?
If you didn't work the entire year or your income was otherwise variable (bonuses or whatever other reason) then it makes more sense than usual to do roth contributions, or rollovers/recharacterization from traditional to roth, in lower-income years. Assuming you can afford to, and want to maximize what you put into tax-advantaged accounts.
The annual contribution limits are also on dollar amount that goes into the account, so if you're maxing out then you can get more out of a roth ira/401k overall. Not all companies offer the choice of roth on their 401k plans, which is a little annoying.