There are issues when more than robot is using the same type of ultrasonic sensor on the field. They can interfere with each other if they use the same frequencies.
Also, you should implement a circular buffer that takes median readings to filter out noisy/bad reads:
http://www.maxbotix.com/articles/129.htm
We used to use these ultrasonics on our robot with some success, but don't expect really great precision. Often you just aren't sure which surface you are measuring distance to, as the detection cone can be pretty wide. To use this year's field as an example, are you detecting the wall of the tower? back of a goal? lip on the batter? batter divider? hanging bar? any of those surfaces will give you slightly different distance reads.
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