See this thread.If you scan a library of lossless CD tracks with an upsampling reconstruction filter, you will find that most overs are less than +2 dBFS. I haven't seen any over +3 dBFS, and I'm not entirely sure that anything above 3.01 dBFS is possible when the input signal has been properly band limited. In any event, 3 dB of headroom is sufficient.
We used 3.5 dB in the DAC2 converters and then backed this down to 3.0 dB in the DAC3 converters because we concluded that 3.5 dB was overkill. Every dB reserved for intersample overs reduces the SNR of the DAC by 1 dB, so you don't want to reserve more than is necessary. I have not found a CD recording with an over that exceeds +3.0 dBFS, but one might exist.
I've mentioned it before here on this forum, but it's entirely possible to have music that has intersample peaks beyond +6 dBFS, or one whole bit. It was mentioned again later on this thread by @LTig. Here is a 30-second excerpt of "Get Away" from the Electronic Soundtrack album Code Lyoko by...