I am not sure if you need dead accurate birth time of natives for delineation.
Would you say thousands and hundred years ago, they were all recording their birth times of people as accurate as minutes?
I would rather doubt about that actually. Their clocks were based on Sun dial stuff long time ago, and then what type of clocks and watches had they been using? I am not sure, but not that accurate ones, I would imagine.
Really it is a bit too naive to imagine that they were cutting birth times of people to minutes and seconds.
Even people who were born in recent times don't seem to have accurate birth times.
And really, you don't need that accurate birth time to make astrological delineation. Even if you did, depending on what house system you use, planets will change their positions in the houses.