Those are good questions. It all depends on what your focus in the game is. For me personally, I just enjoy the world of Pokemon. I enjoy the storyline, I like beating the game, and I want to raise my Pokemon well in that context. I have grinded in some games in the past, but with Generation 6 I think I am going to take a more laissez-faire approach. I have never done EV training before, partly because I did not understand it, partly because it takes too long, and partly because it requires obtaining special items to speed up the training process, items which require you to jump through hoops to get. Also, EV training is not as important to me because I do not do competitive battling.
With X and Y, I think my strategy will be to build a solid team, enjoy the new experience, and beat the main game. Then I will catch and breed Pokemon to continue my journey with Pokemon.