I read here of a method for cutting the tongues of pre-laid tongue and groove floorboards with a Stanley knife. I've got nothing against Stanley knives, I made my very first coffee table with one, only interrupted by a visit to the casualty department for stitches. :-) I don't find it particularly easy to cut along a floor board edge joint with one of these for several reasons, including grit and tight spots, especially when working close to a wall.
So after some early attempts I decide to sharpen up the end of a triangular bladed filling knife, one of the old type with an integral forged ferrule and rosewood handle sandwiching the metal body. This allowed me to cut easily along the edge of each board, applying just the required amount of driving force with a mallet. HTH