I suggest you trawl through my 'Bowyers Diary' where I've detailed the build of loads of ELBs along with some failures and some posts specifically about your question.
As a rule I support the bow where my the main force will be on my hand which is about centre of the bow (that's the base of my thumb) I pull the string from where my fingers will be. The bow is also allowed to rock on the tiller.
Have a look at this post where I reverse a bow on the tiller.
http://bowyersdiary.blogspot.co.uk/2013/03/tillering-symmetry.htmlYou can go back to where I start that bow and see right through the build, I actually reverse it and the nend up turning it back again.
Quick answer is the lower limbs is generally a tad stiffer than the lower.
Data from the Mary Rose bows also supports this.
If you build the bow completely symmetrical the lower limb will probably appear weak.
The only bows I've made where the tiller shifted after some use had the lower limb go weak.
If you aim to build the bow5# over weight with a slightly stiff lower limb, you can always adjust it after it;s shot in for 100 arrows or so, if you build it spot on and the lower limb starts to look weak you'll end up way under weight by the time you have corrected it.
Del