The best time to summon in terms of getting the most bang for your orbs are on legendary banners.
They have better odds at pulling 5 stars and every 5 star pulled on the banner is guaranteed to be a focus unit. The focus units on these banners generally are the best and most sought after units in the game.
There won't be another one for a while though.
Generally, use your orbs to get specific characters. If there is a character you really want, pull on their banner and summon their colored orbs.
I generally don't only go for the best units. I like collecting the babes and just characters I like from the games.
It so happens though that a lot of the babes and fan favorite characters are also top tier units.
I've yet to find a better mage character than reinhardt. ---