Mostly depends on what you want. There are plenty of units that can have different builds, so while Steady Breath dragons is super common, you can use Steady Breath on someone like Arthur, or Bartre, to make them good units.
Tbh I would hold onto that extra Ike anyways. You never know when you'll pull a great unit that works well with SB. Better safe than sorry.
I could see SB on Lukas, but I would only do that if he was a merge project.
Currently, the Lukas is a 4* supermerge project. He's sitting at +5 now. Feathers have been scarce for me, so I have resorted to 4* merge projects for some units.
I'll hang on to the Ike in the meantime, though. Thanks for the advice.