Treasury Secretary James A. Baker III said that President Reagan remains firmly opposed to any new taxes and that any legislation to boost taxes could lead the nation into a recession. In an address to an international conference of tax experts studying "Tax Policy in the 21st Century," Baker repeated the Administration's belief that the country's huge budget deficits have been caused by excessive government spending rather than by Reagan's opposition to tax hikes. "This Administration opposes the call for higher taxes in the United States. We don't want a tax bill to become a recession bill," he said. "Moreover, politicians always find it easier to raise taxes than to cut spending. And even talk of raising taxes tends to dissipate any spending discipline in Congress."