The function \( f(x) = ax^2 - b |x| \) involves an absolute value function.
- When \(x = 0\), the function simplifies to \(f(0) = 0\).
- If \(a>0\) and \(b>0\), the quadratic term dominates, leading to a minimum at \(x = 0\).
Thus, the function is minimized when \(a>0\) and \(b>0\).
\[
\boxed{\text{Minimized whenever } a>0, b>0}
\]