I doubt if it matters.Some mount them horizontally,some vertical.When I started,the theory was,the horizontal blade affected things less when going through paradox.I still mount mine that way because I'm used to that "sight picture" for lack of a better term.
Hot water isn't enough heat to soften epoxies.I don't like heating carbons at all.The best way for me,is get it done when gluing in the insert OR use a glue in broadhead with adapter and line up the blades when screwed into the shaft and gluing to the adapter.