Here's the link to one thread on the subject, though not the main one.
http://forum.1cpublishing.eu/showthread.php?t=4560
The argument boils down to this:
-Leadership bonuses start out small and get progressively larger as you go up in levels.
-Bonuses from Attack, Defense, and Intelligence are good at all levels
-Therefore, to be strongest at the end of the game, go for stat bonuses the first 15 levels, and leadership bonuses every other level after 15.
My own experience suggests the decision is not so straightforward. While gaining 80 or 100 leadership is negligible at the end of the game, it can matter a great deal when you're low level. If I skipped all leadership upgrades early on, I found the mid-game was much tougher than I was prepared for. Also, I found that having a few extra Attack or Defense was not critical at the end - far more important was the makeup of my army, what spells I had, what specific rage abilities I had improved.
Of course, if I find a couple leadership boosting items, such as batons, early in the game, I can ignore the leadership bonuses until later. But if I don't, I prefer to take leadership every level it's offered, and select my other bonuses based on where my character is weakest.
Basically, it's a matter of personal style. Go with what works for you.