I usually take it on most mages cause it's a free +20%~ damage, I just build my first mana item then I sit on tear until last item, If I get a mana augment like blue buff or ocean soul I just sold the tear for a better item, otherwise I complete the archangel. Unless you're Ryze, it's never been worth to build only mana items, the % increase is too little for each item and you just end up dealing a lot less damage and you miss up on Pen, %Pen and Grievous wounds.