Our ability to differentiate between volume envelope shapes depends on how fast the change is. If the change is short, less than 20ms or so, then you can't really hear any difference. Just use linear curves for those. You only need to get fancy with automation curves when the transition happens over a long period, more than a second or two in duration. Even then, it's unlikely that it needs to be anything fancier than a combination of slow and fast curves.
A useful fourth slope option is the "S" curve. That's where the level rises quickly in the first portion and then less quickly for the second portion (or vice versa). It's not a built-in option, but it's very easy to create such a curve by simply adding a node to split the curve into two parts.
Between the slow, fast and "S" curves you've pretty much got it covered. These are the most natural-sounding curves that most accurately model amplitude changes as we hear them in the acoustical world.
For those rare occasions when you really do need a fancy curve, build it out of slow and fast segments (or draw it in, although I've never found it necessary to do that, and believe me I am Mr. Automation!).