Going up and down the strings at an angle is natural, and it may be more or less pronounced depending on picking style.
About where to pick the strings, that depends on you, on what you want it to sound like, and on your style and technique.
Minor changes don't make much difference, but if the position changes a lot, so does the sound you produce.
Picking very near the bridge is usually avoided, or let's say, reserved for particular effects and techniques.
Picking very near or even above the fretboard is also usually avoided and only used for particular effects (e.g. wide strumming motions)
In between those extremes, picking closer to the bridge may result in slightly easier picking when going fast, because the string snaps faster around the pick or finger.
The sound will also slightly change, being brighter and harsher near the bridge, and mellower toward the fretboard.
Experiment, listen to the different sounds, and then settle for what you like the most, and what you find most convenient position-wise.