I'm a little confused on the question, so I'll give two answers --
1) You always want to record in 24-bit -- It allows for a wider dynamic range and there isn't the buildup of dither noise that would be present in 16-bit recordings. Calculations after that are probably going to be in 32-bit or higher.
2) Exporting to 16-bit is fine for a finished project -- Traditionally, once a project is mixed, it isn't yet finished ("traditionally").
That said -- I understand there are units out there that record in 24-bit but will only export in 16-bit CDA. I have several clients that send in stacks of discs for every project because they can't just export a collection of 24-bit PCM files (some of them, I've talked into getting a Masterlink or something along those lines or simply mixing a 24-bit PCM stream out to a simple computer interface).
Gotta say I never quite understood why...