Almost always, Faction Guardians. I would need very specific cases and even then it usually needs to be a champion you can +2. 

Almost always better to get the bonus that applies to the whole faction.  As Thorne mentioned empo doesn't have a really big effect until +2 anyway.

I have 3 Harvest Jacks. Wait with the third to get a forth? Or unbind? Have all the champs there a available to get by token trading. Plarium could put more champs to buy with tokens.

If your FG isn't full just hold them.  It's annoying having 3 but you'll be happy to have saved them if you pull a 4th.  Especially with the powerhouse faction that is Undead.

How do you get tokens anw?

You unbind a dupe lego

The real answer is: you don't. Barring extreme exceptional situations (eg you *absolutely* must have fu-shan for a CB team that will take you from 3-keying unm to 1-keying it and you have no other option for the absolutely insanely long future), there's no reason to ever sacrifice legos.

It's a trap.

Interested post for me just now.

to answer op,

1) always fg until fg is full

2) once fg full, is it a champ you use consisgently in multi content => empower; or is it a crap leggo => unbind.

you ask specifics about 'others to benedit' but they will come.   You will never get all 5 slots filled if you keep empowering/unbinding as soon as you get a dup.

personally I've never unbound a champ, and have only empowered one. i sacrificed 4 nobles to get a +4 ninja (good trade no regrets. (I think I may have previously said it was a +4 Rotos but I miswrote)).

now for the part that makes this interesting to me, just this week I finally filled my first fg.  Shazar, Drex, duchess, Helicath, hephrak.

so now no need to keep dups for fg.  I'll build my second heph, would build a third duchess or Helicath.  But any other dups would depend if I empowered or unbminded.

Thx for all the advice. Will do just so