the orientation does matter with some pickups. Straight from dimarzio.com
Does it matter which direction I install my pickup?
Most humbuckers are symmetrical and their sound does not change at all when installed backwards. Humbuckers that are not mentioned in this FAQ sound the same when installed in either direction.
Some humbuckers have a distinctly different sound if you turn them around. There is no right or wrong installation direction. The Bluesbucker® is brighter-sounding with the adjustable screw coil toward the bridge. The D Sonic™ and Crunch Lab™ models (6 and 7 string) and the MegaDrive™ are brighter-sounding with the solid bar coil toward the bridge. The Steve’s Special™ and Blaze™ Bridge model are brighter-sounding with the cable exit side toward the bridge.
Some humbuckers have just a slightly different sound if you flip them to the opposite direction. There is no right or wrong orientation, but it’s a matter of preference for such a subtle difference, if you can hear the difference at all. Models that sound slightly brighter with the cable exit toward the bridge include:
D Activator™ Bridge
D Activator™ Neck
Models that sound slightly brighter with the cable exit toward the neck include:
Air Norton® 7
DLX Plus™ Neck
Humbucker from Hell™
Tone Zone® 7
Single-coil size humbuckers with side by side coils have coils that are so close together that reversing the installation direction does not have an audible effect on the sound.
there you go.
Second way is that (at least on my Dimario's) it states which side is F= forward=north on the the pickup bottom plate. This should be forward towards the headstock.
The F on the base plate means that the pickup is F-spaced and is not an indication of how it "should" be installed. It simply means that the pickup has a larger space between pole pieces in order accommodate a wider string spacing like that found on some fender and floyd rose equipped guitars.