The best summary on this topic is perhaps contained in the words of the scholar Abu-l-Khair bin al-Jazari (d.833/1429), who wrote:

 'Every reading in accordance with Arabic (grammar) even if (only) in some way, and in accordance with one of the masahif of 'Uthman, even if (only) probable, and with sound chain of transmission, is a correct (sahih) reading, which must not be rejected, and may not be denied, but it belongs to the seven modes (ahruf) according to which the Qur'an was revealed, and the people are obliged to accept it, no matter whether it is from the seven Imams, or the ten or from other accepted Imams, but when one of these three conditions is not fulfilled, it must be rejected as weak (daif) or exceptional (shadh) or void (batil), no matter whether it is from the seven or from one who is older than them.' [Suyuti, Itqan, I, p.75 ]