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While it may not be unsafe (won't blow out or roll off the rim), the only real advantage is looks. Do what you wish of course, but if it were me I'd not run a tire on a rim narrower than it is speced for.
Since no one else answered this question I will. Mounting tires on rims narrower than they were designed for forces the tread to "bulge" in the center. Depending on the tire and the air pressure you run, this can reduce the contact patch and reduce traction. It can, and often does, result in premature wear on the center of the tread, shortening tire life.
While it may not be unsafe (won't blow out or roll off the rim), the only real advantage is looks. Do what you wish of course, but if it were me I'd not run a tire on a rim narrower than it is speced for.
