I use the padlock to save thinking time and prevent accidental sacrifices. I do have to scroll down to select my champions, but once there, I stay there until done or until I'm ready to go through some battle reports before restarting. I only lock champions I do not not wish to sacrifice or champions that will be used in the future to enlighten my Rank 5 or Rank 6 champs.
I find this easier, as I don't have to take time to think about whether or not I was saving a champion, since if its not locked, its safe to sacrifice. Plus, locked champions do not show up on the sacrifice screen, anyway.
In the beginning, I would max out every champion before sacrificing it to get emeralds. Now that I've maxed enough Rank 4 and 5 champs, I have enough emeralds that I don't bother maxing out Rank 1-3 champs anymore unless they're going to be upgraded. Its seems faster anyway (IMO) to sacrifice them for rank upgrades as soon as they're level 1 of the needed rank rather than waste energy and time maxing them out.
I imagine the filtering option that works best will be different for each player depending on how their brain/mind works to solve various problems. There is no overall right or wrong way here, i think. I used various filtering methods, and settled on rank as what works best for me, so far at least.