Because the rifle is 1/2 MOA now doesn't mean it will stay that way if you change stocks, because the pressure points between the action and the stock will change. This will change the dynamics of how the rounds fired will react.
Of course it doesn't mean that it won't perform just as well, but on average, a change in stock can change performance.
Changing from a factory stock to more of a customized stock would benefit from some sort of bedding, but it does not necessarily have to be pillar bedding. It can be standard custom bedding (glass bedding) to create a hand in glove fit between what sounds like one hell of a good action and barrel to your new stock.
The easiest way to understand what this does is to basically create a rock solid platform for the action to sit on, so there are no external harmonics that can change between fired shots.
If you have questions one of the best things I can recommend is find a good, local gunsmith and have him/her take a look at your specific rifle.