The propagation of optical pulses in nonlinear waveguide and directional coupler structures consisting of linear and self-focusing media is studied numerically. Instantaneous and integrating nonlinearities are considered. A comparison of the stability of waveguide modes and coupling characteristics under such conditions is made. Finite medium response times are shown to lead to an increase in the stability of pulses propagating in waveguide structures. For directional coupler geometries it is shown that finite response times lead to a loss of definition between the cross and bar states as the coupling efficiencies of more intense pulses are increased. These effects are found to be dependent on the initial, spatial, and temporal pulse profiles.