Do I need to fix my underbite before getting an impact?

I have a broken front tooth (lower jaw). It probably should have been fixed with a crown years ago, but my dentist just reconstructed it the best he could at the time. I want to get an impact for the tooth now, but my new dentist wants me to correct my underbite first as he feels the impact may be compromised due to the cross bite later on. The cost of both is way over my budget for the time being. Do I really need to fix the underbite?