Steep downhill = straight
Slight downhill = straight
Level = straight
Slightly uphill = straight
Steep uphill = angled
These are my preferences
To each, his own.
I would add that for a position target shooter the angled is also best as one inlynhas to move their head a little to see the hit.
Phil,
I agree with all you've said, and will add that for practical-style shooting and competition, with multiple targets at different locations, I've found straight to be preferable since it allows the user to switch between targets quickly and easily. If a guy attends some PRS-style matches, the spotters he sees will be almost exclusively straight, IME.
