The time of birth should be reasonably accurate. It is a good idea to assess the accuracy in the time of birth when giving it to an astrologer (like +/- one minute, +/- 5 minutes or +/- 10 minutes etc). I, for example, never assume that the data given is right. I consider various possibilities within the given error and, comparing the astrological indications with past life events, I figure out the right time of birth. But, if the original error is too large, I may be helpless.
Sometimes, an error of 10 minutes may not have a major impact on the horoscope, but sometimes even an error of 3 minutes can change the chart a lot. It all depends on the individual case.