I literally put my tuning machines or tuners on upside down AND backwards on my first build. I had no clue there was a right way. After a comment clued me in, I went to Guitar Center and observed that their was a pattern that was always followed. The objective is to have them installed such that the string tension pulls the shaft into the gear instead of away from it. I am using inexpensive Ping Economy tuners and if I put them on right they stay tight. Backwards works ok but there is a noticeable difference. Finding a balance of useful parts has driven me to intentionally put them on upside down to not waste tuners. I also flip em any way that works on my didley bows. I have noticed in the 20,000 + pics on this site that tuners land on necks lots of different ways so I would say this fits the NO RULES rule.
What if there were no hypothetical questions?
Regardless of rules or not I would like to do the best I can and have the machines working correctly if possible so I have learned to follow the standard guitar builder protocol. It is shaft towards the bridge and knob shaft up which is easier to type than to visualize for an old dyslexic like me. It still have to draw myself pictures to get 'em right. I also made patterns out of card stock so I could do standard layouts without twisting my thinker.
So here is a photo essay to help show the difference - its pictures of four of my builds.
Left side is correct notice that the knob shaft is above the gear. Also in the playing position the knobs all turn in a logical direction and the two upper strings are controlled by the upper knobs. The guitar on the right is the very first one I ever built and it is literally upside down and backwards. It works great !! Not loose or anything. I had an early comment about it that ask if it was left handed? Which caused me to figure all this out. Thanks Ted.....
Back of neck - string pulls down on the geared shaft thru the neck and tends to tighten the mechanism on the left side picture. On the right side example it tends to lift the gears apart. Correctly installed its always adjusting itself tighter - backwards works ok because most of our stuff is lightly strung. I suspect its a bigger issue with tighter tuning like Dad strumsticks and such? Just guessing on that.....
Top correct and bottom upside down. See the gear shaft thru the neck? String tension wants to pull the gears away from each other.
Top example is correct and bottom is upside down. This is the three in line which I decided I didn't like. It requires something to hold the strings down. Might be alright with more head angle. I made it work ok - its just not as simple and elegant as some other schemes. KISS ! I have gotten pretty used to glancing at these now and seeing which is right - the shaft with the knob needs to be up - not down.
Bottom is correct and top is flipped. Notice the knobs are higher on the back than the string end? Also didn't get all the bandsaw teeth marks sanded out of the top one... Argh! Sometimes I wish that camera wasn't quite so good.
Another shot of my first one upside down and backwards - simple to see that the knobs are down in relation to the shafts. Works fine and holds tune well - Built it in August of 2009 and played it lots. I do not hesitate to put them on however I need to but I guarantee that a trip to a guitar store will show that ALL the commercial guitar tuners are installed one way. Fortunately for us we have lightly strung stuff (mostly) and NO RULES
Ain't it great !! Bill