Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background Radiation, in particular QUaD1, the PLANCK Surveyor2 and MBI 3. These telescopes contain a range of quasi-optical components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble polarisation signals. Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element optical systems. In general there is a lack of dedicated software tools for modelling the range of components and propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house software packages for this task. We describe the techniques used, their predictions and the performance of the telescopes that have been measured to-date.