Considerations for placing STL on build plate
Orienting items
Orienting items on the build plate should consider:

overhangs - many hangning fragments will need additional support to be printed (cost of time and material, risk of failure); gravity and colling process will affect loose/hanging plastic in this or that way
layered structure - parts will be stronger in X-Y directions, and weaker in Z direction
adhesion - it is good to have wide foot to touch and stick to the bed, then only thin base growing big upwards, which may easily fail by any accident
space - if the thing is big, it may have to be rotated diagonally to fit the printing surface

So most of it comes from particular shape. Try to look at thing from different sides, spatial awareness helps a lot.
Placing items
Also, placing items on the build plate should consider:

heating system - heated bed surface may be warmer at center; even if the bed is heated evenly, than surface will cool down faster at sides, which may affect adhesion (e.g. ABS will wrap more often)
cooling system - sometimes printer has better cooling from one side (depending on location of fans or construction of cooling ducts), so any overhangs (fragments which have to cool down very quickly) should be properly exposed, otherwise they will wrap
environment (external influences) - they may affect heating / cooling as well; also mutual position of few objects may affect cooling or act as wall for wind
printer construction - e.g. backside of Prusa Air 2 frame is shrinking going up, and anything mounted behing extruder will hit the frame - so high prints should be X-centered/narrow, and rather wide along Y axis
multiple extrusion - if you use dual extruder, then mixed fragments may not be located on edges, because one of nozzles just cannot reach it
surface quality - e.g. I print ABS on Kapton tape; sometimes the tape gets small holes or tearing from hotend accidents or forceful part detachment - then I just avoid damaged areas
other issues - e.g. my Y axis is louder than X, therefore I rather try to use X axis more often (especially when printing late evenings)
view point - easthetics of printing: which objects, which sides you want to observe during printing?
acessability - if you may want to influence area during print, put something inside the part during printing?

So most of it comes from the experiences with particular printer - its technical parameters and observations. Many times it is only a choice - like simple PLA prints sticking well.