Round the coordinates to the nearest whole number and then conditionally add 0.5 pixels if the line width is odd. This makes it so the crosshair is never drawn in between screen pixels and so it will always look crisp/clear.
This is basically the same as #17, which I should have merged a while ago, but this implementation is a little nicer and more robust. Since this is pretty straightforward I'm going to merge this for 0.8, then later pull in the rest of the other one, which involves sub-pixel fixes for the selection plugin.
Thanks for submitting this!
On second thought, the other implementation is nicer in that it applies the offsets only when drawing, rather than actually changing the crosshair position. That makes more sense to me. If you'd like to update your implementation, I'll merge that; otherwise I'll take another look at this later this evening, and probably end up taking some of both.