This is true for quadratic polynomials with constant term +1. Any such polynomial is the determinant of a matrix in SL2 ℤ (e.g. using the companion matrix as indicated by Ben Webster). It's well known that
any such matrix is conjugate to a multiple product of
[[1,1],[0,1]] and [[1,0],[1,1]] (upper and lower triangular unipotent matrices).
I'm not sure the original reference for this fact, but a reference is
Proposition 2.1 of this paper.
I believe that your criterion implies that the maximal root of the polynomial is a Perron number. If so, then Lind has shown that every Perron number occurs as the spectral radius of a non-negative integral Perron-Frobenius matrix (and therefore the spectral radius of a recurrent digraph). This only implies that the polynomial divides the characteristic polynomial of the matrix - there might be other factors.
The general quadratic case might be possible to work out using Markov partitions of the induced map of a torus.
I forgot about the cyclotomic case, which can occur if the matrix is not Perron-Frobenius. If the polynomial is irreducible, I think the condition implies that the maximal norm roots are complex Perron numbers (or cyclotomic). These crop up in work of Kenyon on self-similar tilings (MR1392326 (97j:52025) ).