If you purchase softwood 4 X 2s, 6 X 2s, etc from any builders merchant, Homebase, B&Q, etc, it will have been dried to approximately 18- 20% moisture content, and is likely to be somewhere near that MC when you buy it. C16 dry strength graded softwood is the most common grade employed for general purpose construction work. In an unheated shed or garage, or one rarely heated, your installed wood is likely to hover between about 15 or 16% and 20% MC depending on the season and how well the building is insulated and waterproofed. Therefore it would make sense to install wood that's somewhere near those MC levels.
Of course, you could use woood that is either drier or wetter, but that 15- 20% MC range is what your wood will end up at anyway, so you may as well start with that. Slainte.