I keep thinking about this and cant make up my mind.I would think the pores would be exposed more in a horizontal stave, making for better glue joints. That's just a guess.
End grain is vascular. That's why it absorbs more when you paint/stain it. It does the same thing with glue. Whether or not this makes for a stronger joint I dunno. Part of me says yes because the glue penetrates deeper and makes a bigger joint. Part of me says no for the same reason, because it makes for a thinner glue layer in said joint.