Not sure if this question has been asked before, I'm just curious because I'm going to make my first custom, how bad must a pony be for you to decide it's a bait? How far are you willing to go to save a pony?
As a G1, G3 and G4 customizer, I can tell you one thing:
It depends
*confetti falls from ceiling along with a sad trombone in the distance*
I tend to customize G1s that have had cancer, pindot mold, or HQG1Cs. Any G1 that I want to keep in my collection that is in very good to mint condition I DO NOT customize.
G3s and G4s are the ponies I have no shame in customizing, no matter their condition. I don't think of them as dearly as G1s, so to some I am sure this seems callous. I have a few G3s and G4s I don't customize, but most are fine to bait as is, imho.
This kind of thing is very subjective. Everyone here has a different idea on what is/is not rare or precious to them. I think if you're going to make a custom, you need to do this based on your experiences and preferences, not necessarily on what we say.
I hope your custom turns out beautifully, no matter what you choose to use as its base!