I feel that that Every level above 30 should yield an Heirloom point, considering that at 31 (1 heirloom point), it's already double the xp from 1-30, and from lvl 31-32, it's .... well.. I don't know how much xp it is, but I'm assuming it's more than 4.6 million.
I always associate heirlooms with time investment whether it be so many hours to get enough xp to get to 31, or so many hours to get so much gold to buy an heirloom.
So 31= 1 heirloom, 32 = 2 heirlooms, 33= 3 heirlooms, 4, 5, whatever.