In any case, training a Pokemon's EV's is always useful when training in a stat that the Pokemon is good at. EV's are similar to natures in a way in how they justify stat changes. Therefore, treat EV's the same.

I like your scheme on the EV training distribution. In my opinion, you should care about your Aggron's sp. def. because he's made for a defensive type. On top of that, its mega greatly increases its Sp. Def. as well. So, I would definitely say it is worth training his Sp. Def..

The only adjustment I think you need to make is with the Atk. and HP. A Pokemon's best EV for one stat is 252 since 4 EV's = 1 stat point (no point going to 255). So far, I think you have a good number on both the Def. and Sp. Def. For the Atk and HP, do 56 EV's for Atk. and 52 EV's for Atk. Therefore, you would have a number divisible by 4, instead of being under (making it useless).

Hope I helped! :)