It's whatever works best, ergonomically, for you.
I spent a fair amount of time in the car moving the seat, the shifter, the steering wheel, etc until I thought I had everything just right for me.
Common mistakes are mounting the shifter too far forward so that the driver shifts briskly into the odd gears a gloved hand hits stuff in the center console, or the reverse case where even number gear shifts cause the arm to foul on the side of the seat (esp the wings on the sides of the seats) or on something else on the console like a e-brake handle. Also, when I see a shift lever that is not roughly aligned with the steering wheel plane, I know that the assembler hasn't paid much attention to the idea that the shifting hand needs to be able to move fluidly from the wheel to the shifter. Look at production cars to get a sense of the relationship of all the pieces here for insight- most of them get close to optimum.
Mounting the shifter on an angle subtly changes this dynamic and may make sense for tight situations. Mine is mounted this way to minimize interference to the tablet in the center stack (hey- it's touch screen, but not that kind of touching!), and retain the ideal shifter-to-steering wheel relationship, and at the same time not forcing my arm to far backward.
It's a delicate procedure, but you will be happier driving the car for the attention paid early on.