Opportunistic networking aims at exploiting sporadic radio links to improve the connectivity of a multi-hop network and to foster data transmissions. Broadcast nature of the wireless channel is an important feature that can be exploited to improve transmissions by using several potential receivers. Opportunistic relaying is thus the first brick for opportunistic networking. However, the advantage of opportunistic relaying may be balanced by energy increase related to having simultaneous active receivers. This paper proposes a thorough analysis of opportunistic relaying efficiency under different realistic radio channel conditions. The study aims at finding the best trade-off between two objectives: energy and latency minimizations, under a hard reliability constraint. We derive an optimal bound, namely, the Pareto front of the related optimization problem, which offers a good insight into the benefits of opportunistic routings compared with classical multi-hop routing schemes. Meanwhile, the lower bound provides a framework to optimize the parameters in physical layer, MAC layer and routing layer from the viewpoint of cross layer during the design or planning phase of a network.