Since when? If that heavy arrow is spined too stiff and adversely affects your groups, then no- a heavier arrow is not more accurate. If your statement were true, why don't we all go get some 2613's with 150gr broadheads? Maybe even slide a 2214 down inside that 2613 to really weigh 'er down.[8D]

The most accurate arrow will be the one that groups best out of your bow. That will depend on your draw poundage, power stroke, cam style, nock travel, foc, arrow length, arrow spine, etc.... It is way too much of a generalized statement to just say that the heavier arrow is inherently more accurate.