Intonation is improved when you shift from position to position, not from finger to finger. We are continually going between these two styles, depending on the musical context. The left shoulder must not lose its mobility. Better intonation and even articulation is achieved when you don't change the angle at which each finger approaches the string. This evens up the vibrato as well.
Intonation is a question of conscience.
A perpendicular hand is more old-fashioned, but it can be helpful for achieving good intonation in the lower positions. The slanted hand can be helpful because, if used throughout the length of the fingerboard, there is no break or difference in sound as you go from lower to higher positions. Adjust your arm position depending on what string you are on. The thumb is good for feeling changes of positions. If you clutch with the thumb, you will lose this sensitivity.