The effect is most likely to be noticed when testing at high temperatures with a 70 degree wedge. Surface waves can drive you crazy. It has to be almost too cold to work to get longitudinal waves with a 45 degree wedge. The more common problem is to be wrong about the location of the reflector because of the change in beam angle. I noticed this problem a couple of years earlier working outside in Wyoming in the winter.
Different plastics used for wedges have different temperature coefficients.
If you keep your angle beam calibration reference at the same temperature as the test object, and keep track of the actual angle and attenuation as the temperature changes, that should take care of most problems unless you really have to have a specific beam angle for a good reason. Attenuation changes can also be due to effects on the couplants. This happens because the velocity change in plastic wedge materials for a given temperature change is larger than the change in the metal. Beam angle increases with increasing temperature. The effect is most likely to be noticed when testing at high temperatures with a 70 degree wedge.