I've been EV training for a while now, but always wondered whether EV points should be more spent on strengths or weaknesses? For instance Roserade has high Special attack and high special defense, but should the points be spent to improve that or should it be spent on the weaker defense and speed?
I know there are different ways to do this, but which is most recommended in that scenario?
Generally speaking, and I mean very generally, you invest in a Pokemon's strengths. Especially if you're playing competitively. For example, it'd be relatively futile to invest in, say, Lucario or Infernape's HP and Defense over an offensive stat and speed.
However, it does get a little tricky when we're talking about defensive Pokemon. Let's use Skarmory as an example. Its base stats are: 65/80/140/40/70/70
Most players will invest in HP and Defense, making it into a nigh unbreakable physical wall. However, it is perfectly valid to invest in its HP and Special Defense as its defense is so naturally high that it can still take physical hits very well but isn't totally vulnerable to special attacks (as before). It can go either way.
Then there are example such as Blissey, who sports a god-awful base 10 Defense, ridiculous 255 HP and great 135 Sp. Def. It is almost mandatory to invest in Blissey's Defense in order to prevent immediate death at the hands of physical attacks. It is redundant to invest in HP, because it's so high. And its advised to invest in Sp. Def in order to maintain high special bulk. Shoring up the weakness is crucial to let Blissey be effective.
Now let's look at your example of Roserade. It has 60/70//55/125/105/90 stats. If you want Roserade to pack a punch, you should consider investing in Speed and Sp. Atk. If you want it to perform a utility role (Spikes/Toxic Spikes), then you can opt to make it specially bulky so it can stay in against special attackers with ease. Investing in Defense won't get you very far, because its HP and Defense are so low. You're better off switching out against most physical attackers anyway. You'd be neglecting Roserade's strengths by putting EVs into Defense or Attack. Its stats aren't like Blissey or Skarmory's; they aren't ridiculous enough to give you much freedom.
That said, there's always room to be creative. You can move the EVs around to hit specific speed numbers, giving you more freedom in certain areas (for fast Pokemon anyway). And you can always surprise an opponent with a unique spread. Just make sure you aren't gimping yourself, because other Pokemon pull off roles better if you're going to neglect their strengths for surprise factor.