http://ddowiki.com/page/Iconic_Past_Life_Feats
An Iconic past life grants both a passive bonus and a past life stance; except for PDK, which grants an action boost (using the boost temporarily disables your IPL stance). You can only have one Iconic past life stance active at a time, but the passive bonuses from all IPL feats are cumulative. So it makes sense to prioritize whichever Iconic you think has the most useful stance; but the other IPL passive bonuses might still be useful, depending on your build. E.g., I'm currently working on SDK lives because I want the Dodge bonus + Ghostly Essence PL stance; after that I'll do PDK for the action boost + extra PRR. Haven't decided if I'll bother with BF or ML IPLs; the extra Fortification & Pos Spellpower would be nice but not necessary and I may well be sick of the ITR process by the time I get there.
http://ddowiki.com/page/Epic_Past_Life_Feats
EPL feats are similar, in that each one grants both passive and stance bonuses. But you can have one EPL active stance per Sphere, which means 4 EPL stances may be active simultaneously, presuming you do at least one ER for each Sphere. So if & when you go for
Epic Completionist, it makes sense to figure out which stances are most useful to you and focus your EPLs on those feats. If you then decide to do another 24(?!) ERs to max out
all of the EPL passives...well, better you than me.
Ultimately, how you take advantage of the Reincarnation system depends on two factors: (A) which PL feats / bonuses are most useful to your "final" build and (B) how high your tolerance for grinding thru past lives is.
Only you can decide what's the answer to the second part.