Is it absolutely necessary to correct for a temperature drift in the resonant frequency of an ultrasonic transducer?
First, generally your transducer is usually in close proximity to your receiving unit (or if they are one and the same). Thus, any change in resonance is likely to occur on both elements at the same time, so it won't affect their send/receive performance.
Secondly, the speed of sound in air depends on density and thus temperature, so you may want to take this into consideration.
Thirdly, all of this depends entirely on the level of accuracy you require. If you environment is not going to be shifting temperature that much, you can probably get by with estimates based on a median temperature for your environment.