5 axis setup

so i am busy installing a 4th axis on my machine and am thinking of a 5th axis at a later date ( doubtful but you never know :-) )

what do you need to look out for regarding the center of the axis on the A and B ? i mean obviously they at 90 degrees , is it ideal to have both axis running thru the center of the part ? this makes sense to me , but means that the part needs to be offset if its a small part ?

this is for head - table config, trunnion style

i see some of the ebay ones have the b axis way too high and would result in the xyz having to have large travels ? or is my thinking flawed