For a while I've been picking near the neck and have only recently begun picking much closer to the bridge. I've noticed a large difference due to the tension in the strings near the bridge being greater. Is it optimal to pick this way for fluency/accuracy?

I know that different tonalities are accomplished via picking at different areas of the string length and to be honest, I prefer the tone when picking near the neck, but the slack in the strings seems to make it more difficult.

So is either method optimal? Where do you pick?

For reference, I have a strat, and began picking between the middle/neck pickups and now pick between the bridge/middle pickups.
I pick between the neck and bridge pickup. I started doing this early in my playing because I would too frequently scrape the pick-ups when picking above them. I've always liked a low action as well so sometimes it can be difficult to avoid. I've been holding my pick in the same position ever since and I have no desire to change.
It's one reason I don't like guitar's with a middle pickup.. Although I haven't played one in years and years I think my picking style is perhaps a little more aggressive than the average player, so I don't want to start scraping the pickups again.
For some reason when I'm strumming I strum above the neck pickup and I have no idea why.
You're not asking the right questions. Until you understand that, you won't be able to get it right. The question in not where, it's how. You can pick near the bridge or right up the neck. Those are tonal adjustments. If you're picking correctly the location won't matter.
If you look at most kick-ass rock guitarists, they almost all pick near the bridge. The tone is more aggressive and the strings move much less - as you have already pointed out - which has greater potential for speed.

Eric Johnson makes use of the difference in tone between the neck and bridge areas, but usually with just a strum of a chord. He doesn't go to the neck area when he's picking fast.
Depends on the sound you want really. If you want a nice chug then P.M. near the bridge will give you that, although I find sweep picking and most leads sound better played up by the neck pickup. Just experiment and see what sounds best, I don't think there is a "right answer" just a better tone.