Wind values can be provided either way. Historically over the water forecasts were rounded to the nearest increment of 5 knots, at least at lower speeds....viz. 5 - 10, 10 - 15, ...
It depends on what NOAA service you use. If you're relying on links such as
, then more than likely the forecast information comes directly from a field output from a numerical model. Numerical models treat air as a discrete continuum, typically parsing field information onto a grid from which information on variables is interpolated.
For more information, refer to http://www.meted.ucar.edu/nwp/course/index.htm