A question we receive often here at AAP.
There is no one single answer, it depends on the facts and circumstances of your particular acquisition.  There have been a number of Comptroller General Decisions on this very topic.
Luckily, the DoD Fiscal Law Desk Book (see Chapter 3, page 3-21) provides a great and succinct description with links to some of these CG decisions.  The DoD Financial Management Regulation does have more specific guidance if your contract is for renovation and repair project; the desk book has a link to that citation as well.
We know there are many contracting offices that have internal ground rules (e.g. issue the NTP before 31 Dec…) but again, these are just guidelines and it really is acquisition specific.