Does the Die Pad of the POWR605 need to be grounded ?
In a word No. The Die Pad is a standard part of the QFN package but, in the case of the POWR605 it is not connected electrically to the actual silicon in any way. As for heat dissipation (the main function of the Die Pad), the POWR605 does not consume enough power for this to ever be an issue. Therefore, the two main reasons for soldering a Die Pad to the board - better grounding and heat dissipation - neither apply.