Peer-to-peer networks are an increasingly popular solution for the distribution of media content to a large number of users, with limited investments for network infrastructures. The distribution of a real time video stream imposes strict performance requirements such as small playback delays and few frame losses. In this paper, we focus on peer-to-peer video streaming systems with tree or forest content distribution structure and we provide a sensitivity analysis to investigate the impact of three critical parameters - rejoin time, average permanence time of peers and playback threshold - over the quality of the video stream received by users. The study, carried out through simulation, considers a general peer-to-peer video streaming reference model with tree/forest topology.