I own a 1989 OOO-28 and owned a 1980 D18 .Style 28 and up have always had ebony boards. Style 18 shifted from ebony to rosewood boards in the late 1940s probably for cost reasons rather than for tone. I prefer the feel of ebony to rosewood and would love to try a D18 with an ebony board however if you can perceive a tone from the standard D18 that you prefer go for it. There are no absolutes in this process.Every good guitar will have its own flavour.
"Life is mostly Froth and Bubble" Adam Lindsay Gordon