The conventional rules are :
(i) If the insignificant digit to be dropped is more than 5 the preceeding digit is increased by 1 but if it is less than five than preceeding digits is not changed, e.g., 1.748 is rounded off to 3 significant figures as 1.75 and 1.742 as 1.74
(ii) If the insignificant digit to be dropped is 5 than this digit is simply dropped if the proceeding digit is even but if odd than the proceeding digit is increased by 1.
e.g., the number 1.845 rounded off to threesignificant digits is 1.84 but for number 1.875 it is 1.88.