Addressing the balancing aspect:
A +1 ability gain is normally allowed in the regular game at levels 4, 8, 12, 16, and 20 in addition to everything else gained at those levels. One point in ability is equal to 1/83 of a PC's total (possible) ability points. This feat is takeable at *every level* in the basic game, starting at level 21, which is only 1 level higher than 20th, but in the basic game it could be in theory taken a total of 20 times.
Yet there are other epic feats available. If this feat is superior to other epic feats and over-powered, then no one would take other epic feats, instead taking this feat in the basic game 20 times.
Compare, then, Overwhelming Critical. In the basic game, it requires a Strength of 23+ and 21st level. On POTM, it in contrast requires only 17th level and an 18th Str. Yet it must be a better feat than the one I've suggested, or no one would ever take it in the base game. The PC would always choose a stat boost instead.
So if this feat is unbalanced, so is Over-whelming Critical.
Likewise, Greater Weapon Focus, which is equivalent to Epic Weapon Focus in the base game, and here becomes available at 13th level (instead of 21st).
One can have a personal preference for each feat, but one can't reasonably argue that the above feats are balanced, and the proposed isn't.
Moreover, the above epic feats aren't equally attractive to all classes, whereas PCs of any class can benefit from this feat.
Now, regarding the odd / even dilemma. As I said earlier, a standard PC receives 83 stat points maximum over the course of her career. This means if she makes maximum efficiency of her points (never spending 2 stat points for one, for example), she *must* wind up on an odd stat. The way to avoid this is to "waste" a point by spending two points to boost her stats by a single point (such as raising a stat from 14 to 15). However, if she does this again to make her stat even to start with, she will again wind up with her stats odd at the end.
Let's look finally at the end game. The PC hits level 16. She can now have an odd stat that she will fix with her 20th level bump. But that means from 16 to 19 she has an odd stat (which could be for many months or even years of playing). Knowing that she could take this feat at 20th level means instead that she can have even points from 16 to 19...and then boost her points by two at the end (1 with the feat, 1 by the normal bump) and still finish with even stats in everything.
So even if this feat ultimately comes into play only at 20th level, it makes deciding how to allocate points throughout the life of the PC easier in terms of efficiency with an eye toward the finished product.