If you have not already, buy a copy of Herb Sutter and Andrei Alexandrescu's "C++ Coding Standards: 101 Rules, Guidelines and Best Practices." Read it. Recommend it to your co-workers. It's a good base for a local coding style.
In Rule 25, the authors recommend:
"Prefer passing by reference if the argument is required and the function won't store a pointer to it or otherwise affect its ownership. This states that the argument is required and makes the caller responsible for providing a valid object."
"Argument is required" means NULL is not a valid value.
One of the most frequent causes of defects is accidental de-referencing of null pointers. Using references instead of pointers in these cases can eliminate these at compile-time.
So you have a trade-off -- eliminate a frequent source of errors, or ensure understandability of calling code by means other than the function name. I personally lean toward eliminating risk.
