Is there any reason to hoard skill points, whether from leveling up or in the form of the occasional quest reward? Can any benefit be gained by saving the skill points for later, or are they best used immediately?
Although bwarner and Kyralessa are right about the traditional reason to save skill points, now that Blizzard added the ability to reset your points and respec, this is less important. You can invest points as you get them, and then when you are a high level, reset the points and spend them more "optimally." Currently you get one free respec per difficulty level, from Akara in Act 1, and can get more by acquiring certain rare items.
One reason to save skill points is to unlock multiple skills once you get to the level of those skills.
When you get to level 6, for instance, you can unlock more than one skill at that level if you've saved enough skill points (and you have skill points in any skills that directly precede them).
Let's say you have skill A1 and skill A6 (which depends on having A1), and skills B1 and B6.
In level 1 you add skill A1, then in level 2 skill B1. Then you save the next few points, plus the one you get from Akara, so at level 6 you have four points available.
Now you can put one point in skill A6 and one point in skill B6.
Then at level 7 you can put another point in skill A6 and another in skill B6, for two points apiece.
You can still only put one point per level, but you can spread the points between multiple skills if you have multiple points in reserve.
(Or you can just play PlugY, which is all I do anymore.)
Yes, technically it is best to map out where you want all of your skill points spent when you reach max level, and only spend skill points when you can allocate them in accordance with that map. This generally involves maxing out the skills at the bottom of the tree, and then backfilling with whatever skills provide the best bonuses to those "end-game" skills. The end result is that you'll do a few more percentage points of damage at the end game. If you aren't concerned about being completely maxed out, then you can feel free to use them whenever.