Energy efficient and fast yet reliable spectrum sensing in cognitive radio networks (CRNs) is an important issue and is often met through the involvement of (optimal) multiple sensing nodes in collaboration known as cooperative spectrum sensing (CSS). However, due to severe fading, sometimes transmission of sensing samples over some reporting channels (R-channels) consume much energy but contribute very little on global sensing decision. To address the problem, this work explores the scope of using an optimal checkpoint to stop further transmission of sensing samples over the deeply faded R-channels. First an energy minimization problem under the constraints of detection and false alarm probabilities is developed in an amplify forward relay assisted CSS system in terms of optimal sample checkpoint, the number of essential sensing/relay nodes and relay power gain. The problem is further extended in presence of primary user emulation attack (PUEA). Simulation results show that a performance gain ∼77.17% and ∼71.25% in energy consumption can be achieved for the proposed approach as compared to the existing two works at Pd=0.9 and Pf=0.05.