Hook shape is dictated basically by two factors. 1. What shape is the insect you are representing in the stage of life you are representing and 2. What do you need the pattern to do.
#2 is probably a bit easier, so I'll start there. Folks who tie dry flies on scud/rounded shank hooks typically want the body of the fly to sit below the surface film of the water. There aren't many traditional patterns out there that call for this, mostly emergers. For a totally floating fly, like any of the Catskill style patterns, you want a straight shank hook so you can extend a tail/shuck straight off the back of the hook to help with floating it. My advice is to purchase standard (straight shanked) hooks for your dry fly tying.
#1. Different nymphs behave differently when they're in the drift. Some kinda ball up, some go straight, some wiggle, some swim, some contort into a reverse C, etc. This is also dependant on what they're trying to accomplish, such as getting back to the safety of the bottom or getting to the surface to emerge. Even scuds aren't all curved like the scud hook when they're in the drift. When their just floating or dead, they're curved. When swimming, they're usually kinda straight. My advice is again to go with a straight shanked hook for your nymphs.
Hook gap per same size of hook will be larger on a scud hook than the standard of the same size hook. Hook shank length will be different between a scud hook and a standard nymph hook. If you really know what you are doing when it comes to proportions of your nymphs, this won't matter becasue you'll be able to adjust. If you're at the point in your tying that you are relying on some standard recipe to tie your flies, you'll have some problems getting the sizes you want. There is also a difference between the shank length of a standard dry fly hook and a standard nymph hook of the same size. Nymph hooks are longer.
Also note that you can take a standard straight shanked hook and bend it to give you a scud hook profile rather easily. You really can't make a scud hook straight.