It won't hurt if the tooth bucket is a little wider. It will penetrate into the grade much easier since the weight of the machine is concentrated from say 68" down to about 14" across the tips of the teeth.
Just be aware that larger buckets weigh more. If you are trying to stay within the Rated Operating Capacity (ROC), you may not be able use the full capacity of the wider bucket. It will depend on the density of the materials you are working with.
Personally I wouldn't purchase a bucket that was not at least 4 inches wider than the loader. The only reason to go narrower than the track width would be for bulk excavation in difficult to penetrate conditions. With a narrow bucket you are giving up the ability to accurately cut a grade and possibly not using the available capacity of the loader.
Just my 2 cents worth...