When properly done- receiver bedding and pillars will almost always improve accuracy.
Without it, there's a near certain probability that the action is going to move in the stock under recoil, and that's what affects the consistency of the location of the muzzle when the bullet leaves the barrel.
Bedding "locks" the receiver into place-not allowing it to move- while the pillars eliminate stress points on the receiver resulting from contact with the stock.
Both is best, if doing only one I would do the receiver.
If you DIY, be sure to research thoroughly. If done improperly (like clamping one spot on the receiver to the stock while curing), you can actually induce stress into the receiver, and make it worse. Not rocket science, I've done plenty of them and it's not difficult if you've got basic woodworking experience and a Dremel...