Is that generally a good idea? Or will my hero be crappy if I do that?
Depends on you. You're not locked into this decision by the way. You can reclass whenever you want with a Heart seal. You can either promote from you Base class of a Nohr Prince to a Hoshido Noble/Nohr Noble depending on which game you have with a Master Seal. And after that if you want you can Heart seal over to the Oni-Savage's promotes which are Blacksmith or Oni-Chieftain.
So if you find the current class you are in doesn't fit you can just use a Heart Seal again and go back to your original Promotion or try the other one. Once you throw Partner and Friendship seals into the mix you can start to access a wide range of other Promoted classes that your other units have access to.