You have to understand that in Astrology, there are principles which make the dosha. Say Horoscope A and Horoscope B. When we check horoscope A, there is nothing wrong in Horoscope A. Similarly there is nothing wrong in Horoscope B. However when we check the compatibility we see that there are some aspects between Horoscope A and Horoscope B, which do not match properly and likely to give problems. Like we have principles which make these doshas. Astrology has counter principles which negate the doshas. That is known as cancellation rules or exceptions. That is true for every dosha, be it Kuja dosham, or Nadi dosham etc.
Siro Rajju dosha is not good so we need to check if Rajju dosha gets cancelled by these counter principles or not. If the Rajju dosha gets cancelled then marriage can be solemnized without any problems. If the dosha does not get cancelled then some Parihar, remedy may need to be done.