There is no such thing as a perfect speaker, nor a perfect listening environment. Your goal should be to create a mix that sounds great (translates) to other speakers/environments. Having multiple sets of speakers/monitors lets you hear your mixes differently and you may notice things that need to be fixed when listening on different speakers. I typically use my main nearfield monitors for the majority of mixing. Then, when I think I'm close, I'll check the mixes on a system with a subwoofer and a tiny set of computer speakers. If it sounds good on both of those, too, I burn it to a CD and listen to it on my home stereo, portable cd player (with headphones), and in the car. If it's good all of those places, then it's probably a decent mix.
As for upright vs. on the side, it is often at the recommendation of the manufacturer as to how their speakers are designed to be positioned. Also, ideally, the tweater and woofer should be roughly at the same level as your ears when mixing, so some folks will lay down their speakers for that reason. Check the user's manual.
