In commenting on Pat Robertson's suggested response to federal budget deficits, The Post {editorial, June 21} refers to his reliance on "mirror tricks," including "following the specious guidance of the Grace Commission."

According to the Office of Management and Budget, the "specious guidance" offered by the commission resulted in documented savings to taxpayers of $38.9 billion in fiscal 1987. Not a bad return on the $78 million contributed by the private sector to finance the commission's work.

More than $4 billion was saved in 1987 simply from improved financial management -- by expediting the collection and deposit of government receipts, by prescreening loan applicants and by use of private collection agencies and credit bureaus.

Incidentally, implementation of the commission's recommendations has become an initiative of both the executive and the legislative branches: almost one out of every three members of Congress belongs to a Grace caucus.

Over the years The Post has repeatedly denigrated the commission's recommendations. My question is, Why? Does The Post lose its card-ju carrying-liberal credentials if -- God forbid -- it supports an idea that reduces government spending?

Serve, if you must, as apologist/cheerleader for the tax-and-spend team, but at least demand that the money be thoughtfully, efficiently and effectively spent. That is the purpose of the guidance provided by the Grace Commission. JOHN J. GISH New Rochelle, N.Y. The writer was director of special projects for the Grace Commission.