If all you do is the pillars then you can't be sure that the recoil lug is taking all the force from shooting as it is supposed to do.
If all you do is bed the action then you can't be sure the stock isn't being compressed when you assemble the rifle.
If you do both then you have less to worry about concerning the fit of the action to the stock.

When I bed a rifle I do at least pillars and recoil lug. Usually I go ahead and do the entire action if I've gone to the trouble of prepping for a bedding job.