Lithium niobate crystal is widely used for the design and fabrication of integrated electro-optic modulators. As a ferroelectric material, one sees its spontaneous polarization change with temperature variations. This phenomenon, known as the pyroelectric effect, induces strong waveguide transmission variations for waveguides realized on Z-cut wafers. Waveguides made by titanium in-diffusion either on the Z+ or Z- side of the crystal show a significant difference in temperature behavior. Experimental data, enlightened by numerical simulations, help to show why Z- waveguides are more immune to temperature changes than Z+ ones.