I dress up and over toward my left hip, mostly out of necessity.
As a grower, my flaccid arrangement can go either up or down with equal comfort...but I still get random semis and full hardons quite a bit (probably because I'm naked the vast majority of the time and wearing any fabric is a novelty of sensation) and my natural erect position points sharply vertical (45-60 degrees), with my penis itself also having a slight upward curve.
If I were to dress down (I wear boxer briefs) and pop a boner, it would be extremely uncomfortable. In slacks, it pops a tent straight out from my crotch. My 501's are tighter and will contain such movement, but it feels like I'm trying to lift a 4-stone weight with my cock, and the tubular protrusion growing down my thigh isn't subtle.
Dressing up and left allows my cock to expand toward my hip along the crease where my leg joins my torso. While it isn't as visually impressive to the casual observer, it's a damn sight more comfortable for the owner.