Everything Cef explained just above is precisely how I do it to.
Heroic souls get opened if POM frags are on offer (me not having POM Eddie) but otherwise I do not touch any souls above a normal one until the monthly Sac event.
To get the ultimate sacrifice today I had to power up to 100 7 x 5* duplicate toons some were G/A souls and then I had to throw a skill shard in each one to get me over the 50,000 essence mark.