You have no choice.
You need to show a prior years basis on the 8606 of the year you do the conversion.
For that you need a previous year 8606 that shows your non-deductible contribution (s) . (otherwise your 8606 history is inconsistent, which. as far as audits go, might be more likely to cause an audit).
If you don't convert all at once, only a prorated portion of your basis is non-taxed and 8606 will do that calculation also.
Or if you never convert to Roth, without the 8606 the IRA contribution will be taxed twice when you withdraw it.
