Hi,
At a generic or very high level slight inaccuracy in birth-time may not matter when somebody is looking at just your Rashi or D-1 chart (unless of course your Ascendent or Udaya Lagna is near the cusp or border of two Rashis and changing Zodiac signs or Rashis, within the inaccurate tolerance band).
But then, there are other charts (like Saptamsa or D-7, Navamsa or D-9, Dasamsa or D-10 etc.) that are cast/looked into for various specific areas of life. They will vary considerably with small changes or inaccuracies. So, the reading for specific aspects can and will go for a toss.
All said and done, I have witnessed some elevated souls predict simply looking at the Moon/Chandra Lagna and a few blessed souls who seem to have an ability to predict without any birth-charts (they seem to rely on their own intuitions, innate gift, face reading and other such techniques). Ultimately, I believe one should be blessed to come across such noble souls.
Take care…