Almost always on top. I have seen some the other way, but they'll come out too short.
There's so much good information on this site that it's worth reading through.
Strong joint, only in our dreams!
A few of my tips are to put the thing on with a little bit of Elmers. That seems to hold the placement better, then go back and bleed in some CA with a toothpick tip. Or, not.
Second, I always manage to get a corner into each section.
Both of these help avoid having railing sections topple over into puddles of CA, which really sucks.