I strongly disagree: shaving diagonally is the way to go!

It can be tempting as a mustache first-timer to grab your trimmers and shave a horizontal line at the creases of your mouth.
While Tom Selleck is undoubtedly one of the all-time-greatest mustache-wearers, his chevron style takes a lot of time and growth to perfect. For most mere mortals, shaving diagonally down towards the crease of your mouth will create a much more natural look.

That’s generally a much more attractive angle for the mustache to then build up that strength and weight, which is often where guys go wrong, because they push everything up into a straight line, leaving a very clear horizontal when really you want a more natural, diagonal droop around the mouth.