Re: Seymour Duncan Invader Pups
If it won't turn, leave it. Those screws in the pickups that you see are for adjusting string to string balance in very fine increments. For example if your open A string is VASTLY louder than you open D, then you'd possibly start messing around with that.
Beyond that, the stock positions are more than fine for most pickups, and you should worry more about the height of the pickup as a whole, rather than each hex head. Its definitely an 'if it aint broke, dont fix it' kinda thing, although if you're seriously anal about string to string balance, you can use a level meter and your own ears to guide you to a good balance between strings (Although its a lot of hassle and I personally dont hold much truck in it.)
As for string pull, the invader is basically string pull incarnate ANYWAY thanks to the gigantic magnets and superlarge ferrous polepieces. Adjust the pickup to a height that sounds good to you and you should be good to go. Those pups are designed for output over all else anyway, so they're mostly for high gain settings where the distortion itself will sustain notes far longer than you really need.
Alternatively, if you're worried about it, consider a less all out pickup choice. The Evo bridge model is fantastic in every situation. Or if you REALLY want balls to the wall heavy, with a sweet side, then the Dimarzio D-Activators are hands down the best I've heard for that.