J. Gregory Ballentine, newly named partner in the Washington National Tax Practice of Peat Marwick, one of the "Big Eight" accounting firms, has never been one to follow the pack.

As deputy assistant secretary for tax analysis at the Treasury Department, he helped draft the 1982 tax increase -- anathema to much of Treasury's top brass and other Reagan administration supply-siders.

As chief economist at the Office of Management and Budget from 1983 to 1985, Ballentine, although a devout Stockmanite and self-described "fiscal conservative," nonetheless was not one of the "regular line guys" and "a bit of a floater."

And now, as the lone economist in Peat Marwick's tax group, he is the only one of 16 pencil-toting partners whose job jurisdiction is not defined by an IRS tax-code number.

The tax group's partners are highly specialized accountants and lawyers following the most current developments in the tax code primarily for other Peat Marwick offices, said Ballentine. "It's like having your own little dictionary," he said, describing the function of the group in the larger company structure.

In one sense, Ballentine, 37, who has a doctorate in economics from Rice University, is the generalist in an office full of specialists. As an economist who has crafted tax law for Treasury and forecast and monitored the national economy for OMB, he is the firm's resident expert on the full range of government economic issues. Ballentine said he finds that diversity the most interesting aspect of his new job.

Ballentine will help clients, the majority of which are major banking and financial interests, interpret and evaluate "the economic effects of proposed legislative and regulatory changes, particularly as to how these changes would affect them and their industries," said Richard M. Glennon, managing partner of the tax group.