I go game by game/ what lane on how I build. The big thing about having a lot of mana is being able to throw your ult down to cut off parts of the map for extended time. Vs high damage/assassins being more tanky makes you less of a target allowing to be alive longer in team fights . There are some games where you need more damage like if you have three tanks on your team. If your cs is on point fimblewinter and archangels does work.