Ah yes, unconnected systems and time sync... these were the bane of my existence for several years with one client in particular. Nobody ever cares enough about time sync until something happens and then they can't find the video they want because the clock never matches their paypoint systems (which DO have access to a time server). And a big fuss is made about it at the time, the problem is explained AGAIN, solutions are offered AGAIN, and by the time someone decides it's not worth the trouble or expense, nobody cares about it any more anyway. At least not until the next time. Same goes for cameras being down or a system beeping incessantly for weeks because a drive has failed - everyone just ignores it until they need some video, then OF COURSE the video they need is on the dead drive, or should have been captured by the dead camera.
There was one instance with a lottery fraud investigation, where the lottery people were concerned enough about accurate time sync (matching the video time to the time on the lottery terminal, so they could ID the person buying tickets with a stolen credit card) that they brought me in to spend time with their investigator - he'd print a ticket, bring it back to me, and we'd compare the timestamp on the ticket to the time on the DVR. Did this several times, in fact... then he had me write a two-page statement on why the time was wrong and how we determined the proper offset. If you can use a method like this to prove time offset, there should be no problem in court.
Of course, none of this is new to DVRs - part of our annual preventative maintenance on the ol' MUX-and-VCR systems was correcting the time on the MUX, which was always off by at least a couple hours from the previous year... and those didn't even have network time sync as an option. Most didn't know how to handle DST either.
As far as camera vs. DVR timestamp, we've always used the DVR-based one - if the camera has onscreen time display, I'll generally turn that off entirely. In the lottery example above, they originally called me in because the cameras were showing a timestamp in 1970 - they were IQs and the installer hadn't set the time or disabled the onscreen display, and the investigator kept fixating on that instead of the DVR's time display in the playback window.
Keep in mind that camera timestamps really only apply to IP cameras, so if you still have an analog or hybrid system, you still need the DVR's timestamp. As you move from analog to hybrid to all-IP, it just makes sense to keep that time source consistent, especially if you're still using the same VMS.