NEW FRIEND OF BILL: Many Democrats believe President Clinton and his green staff have lots to gain from their growing relationship with a crusty pro, House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.). "Rosty" once was suspect among Democrats because of his friendship with President George Bush. But after a recent four-hour private dinner in Chicago and another strategy session with Clinton in New York, he clearly has a new hero. "We've got a President who is leading," he says. . . . Rostenkowski pushed most of Clinton's huge tax bill through his skeptical committee with amazing speed and is likely to become congressional point man on Clinton's health care plan. And Clinton may be able to help Rostenkowski, who is under federal grand jury investigation for possible misdeeds involving the House post office. . . . Still, Rosty is no toady. When the President suggested a golf game some weekend, the House baron replied: "Are you going to be in Chicago?"