Is it possible to turn of only the grid in orthographic 3D Views?
Assuming you have 'Grid Floor' turned off as moonboots shows, there's a quick trick .. when in one of the standard Orthographic Views, tap 46 on your Numpad..(rotating your view by + and - 15 degrees). This converts your standard Orthographic View to a User Orthographic View, from exactly the same angle, and the ortho background grid disappears
Usually you open up the properties panel N and deactivate Grid Floor under Display. However, toggling that property is not supported for orthographic views. Your best bet is the activation of Only Render within the same panel, which basically hides all overlays and visualisations, see the manual.