So I have some random ideas floating around in my head about this...
1) The first is I agree with the sentiments ahadabans expressed in one of his posts: basically that, sure, the old "hey don't use hard and fast rules" is nice and all, but I think starting with a fixed guideline for a mana curve makes the design process a lot easier. I.e. if I says "I am going to have exactly 7 one drops in white" it is a lot easier to go look for the exact 7 I want then to instead go "hey just kind of feel out your mana curve" then I start having to compare 1 drops to 2 drop and 3 drops and the whole thing just becomes a lot harder. I think starting from a more rigid design process and then being creative after you have at least a baseline framework is easier. For instance I think we all agree that we should have the same number of cards in each color in our cube, but then we also know that once it is all said and done, if we have 82 white cards and 78 black cards its not the absolute end of the world (or maybe some do thinks its the end of the world!). And I think we all agree that it is good to have some fixed guidelines on the the creature to non-creature ratio. At least at the outset. Then as things shake out if you wind up with a few extra or a few fewer creatures than you original intended no big deal. I feel like curve should be approached the same way. As such I see nothing wrong with giving a more precise answer to the question of "what is an "ideal" curve to aim for when starting out" than simply "hey whatever works for you" or "depends depends depends!".
2) I think it does not make sense to have the exact same curve for each color IF you go with the approach that aggro works better in some colors than others. Perhaps some designers believe that you can equally support aggro in every color and also equally support control in every color. I think this is wrong and that it will lead to a poor drafting environment. That discussion is so in depth, though, that it is perhaps beyond the scope of this post. However, assuming you take my assertion axiomatically (that some color support aggro better than others), then I think it stands to reason that curves cannot be identical across colors. I know someone mentioned that "hey a 1-drop is not necessarily an aggro card--plenty of 1/2/3 drops are control cards". This much I agree with. However, what I do not think is true is that plenty of 5/6/7 drops are aggro cards. As such, if you try to make curves across colors identical while at the same time supporting aggro more in some colors and control more in others (taking the original axiom to be true) then you will logically cripple aggro because of the asymmetrical nature of 1 drop slots that can be used to help both aggro AND control and 6 drops slots that can only help control. Bottom line, the construction of an aggro deck is not symmetrical with a control deck.
3) I am not sure the best way to approach mana curve is. One way is to take, say, all 80 black cards and look at the curve. But I think this may be wrong. For it is quite clear that there are certain cards you play on curve (as in you play the card as soon as you have the available mana to play it) and others that you play independent of curve and more reactive to the game situation. Classically, this has usually been viewed through the rather simple creature vs non-creature paradigm. This is because, in general, creatures are something you almost always play on-curve while spells are something that tend to be more off-curve. But I think this only holds true "somewhat". It is certainly a close enough approximation in a booster-draft type limited environment when the power level is low, interactions are more straightforward, and you have a small amount of time to throw together a deck and you are doing a lot of eye-balling. But when you are taking your time spending many hours designing a cube with a spreadsheet program, and designing an environment with a high power level and lots of complex interactions, I think it makes more sense to look at cards on a case-by-case basis and assign each card to one of two piles: "on-curve" (meaning you almost always play it as soon as you can afford) and "off-curve". A card like
mystic snake is a creature but is probably only going to be played when the situation calls for it rather than when you get your 4th land out. The reverse is true for a card like
bitterblossom which I imagine most decks are happy to play as soon as possible even though it is not a "creature spell". And, imo, there are many such examples of cube staples.
Restoration Angel,
Fleshbag Marauder,
Venser, Shaper Savant, etc are all cards that are more likely to be played at the opportune moment as opposed to the earliest possible moment. Thus, I am postulating that it is perhaps more important to ensure a healthy curve specifically for cards that are "on-curve".
4) All this being said, I am still not sure what the right curve is in general, for each color, or for "on-curve" cards. And I am not sure what the appropriate way to evaluate "off-curve" cards is either. Should they have the same curve as "on-curve" cards? Perhaps a flatter curve? Perhaps you shouldn't worry about the curve of cards like that at all? I would be curious to hear people's thoughts on all this.