I don't know if it really matters which way you install them, but I installed my Energy Suspension bushings with the slit facing down. I think that my reasoning might have been that in that position, even though the bushing is closed up tight, that if anything got in there it would then work it's way out away from the sway bar. Hey, I never said it was a valid reason....just my reason.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless