Official time of death is not always stated precisely as evidenced by JFK's official time of death as 1:00pm CST. The priests who entered the ER to administer the last rights before that time said he was already covered with a sheet when the last rights were administered to the body.
The only reason that I know of why a precise time of death might be important is the provisions of some wills. As executor of my father's will, I know there was a provision that excluded the named heirs in the event they preceded him in death. It also stated that if the order of their deaths could not be determined, it was to be presumed they preceded him in death. I think it would be highly unusual that such a provision would come into play. Perhaps if the people question were both killed in a car or plane crash and both were declared dead at the scene. Hard to say from just a few examples but it looks to me like it might be a common practice to round off the time of death to the nearest 15 minute time frame.