You don't need a fisheye to shoot good skate stuff. If you're going to shoot long lens though, you don't have to get super-tight comp. Try to get some of the surrounding area in it while still lighting the skater well. Here's an example I did just two days ago.
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There are a thousand things I don't like about this photo (especially the blown out ground) but you can get the gist of it. Even when shooting quarterpipe-style obstacles, don't give up on landscape. You can get some really col shots that way.
Also, when you do decide to purchase a fisheye, you will learn that the viewfinder is not a necessary tool... one can quickly pull away a camera right after the pic is taken so that the subject can land the trick. Like this!

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As far as dead center goes, just get your weighting right before even taking the shot! But you are off to a good start, I look forward to seeing your new stuff.