The main point of the bicep is supination and this is why bb curls are so effective at emphasizing the inner head of the bicep. I believe that a wider grip further increases this emphasis. I usually go pretty wide on bb curls, about 4-6" past my shoulders. EZ-bar curls, even on the wide grip will not have the same effect as bb curls because of the lack of supination.
For skull crushers I was going closer but it seemed to put some strain on my wrists so I went wider. I dont know which is better, I havent thought or read into the mechanics regarding this one. Here's some speculation: with a closer grip your elbows tend to flare out more causing a greater emphasis to be placed on the outer tricep head closer to the elbow. With a wider grip your arms tend to be more parallel to each other causing a greater emphasis up higher towards the shoulder joint.
Feel free to add or correct me, a lot of what I said is speculation.