Has it been reported that the Companions used to seek Allah's guidance (istikharah) in everything, even in table salt? And are the matters that necessitate istikharah only significant matters?

Istikhara is legislated when one is hesitant between two matters, and is unsure which of them holds the benefit, even if they appear insignificant, provided that the benefit in them is not clear. However, if the benefit in one of them becomes apparent, then Istikhara is not performed. Istikhara is limited to permissible and recommended matters when two such matters conflict. Istikhara encompasses both significant and insignificant affairs, and the fact that the Prophet, peace be upon him, did not perform it for a specific matter does not negate its legitimacy for all matters.