No, because Heatran and Giratina are not roaming Pokemon. The only Pokemon that will roam in D/P are Mesprit and Cresslia, and in Pt the legendary birds will roam as well.
Heatran will also not reappear in D/P, but will in Pt upon defeating the E4. Same with Giratina in Pt.
Bulbapedia - Roaming Pokemon