Is there a "conventional" configuration for mounted accessories? Many different types of bike accessories as designed to be mounted via somewhat-conventionalized braze-ons: These braze-ons most often are on the down tube and occasionally on the seat tube and nearly always seem to be mounted with two screws 8cm apart. However, is there not some sort of accepted convention for the "best" places to put different types of accessories? For example, water bottles are more easily reached while biking if they're on the down tube rather than on the seat tube, so if there's only one water bottle cage, I'm guessing most people would mount it on the down tube. However, what about other stuff?-- Locks, pumps, bananas and so forth?

Case study

I used to have a road bike (a '70s Motobecane randonneur/randonneuse) which had a dedicated braze-on under the top tube for an air pump but nothing else. As explained above, I then mounted a bottle cage to the down tube using a special bracket-braze-on-thingy which mounts to the frame with a metal band and a screw (I wish I knew where I got them from or even what they're called). Then, because I didn't know where else to put it, I mounted the holder for my folding bike lock onto the seat tube using another metal-band-braze-on-thingamajig:



However, after that bike died, I got road bike of very similar style (a '76 Motoconfort (Motobecane) Le Champion). However, an analogous configuration for this bike is actually not possible because it has its air pump mount on the seat tube rather than under the top tube:

 

As you can see, the folding lock is in a very strange place because


hanging it underneath the top tube near the saddle seems like a good place, but then it's not possible to shoulder the bike --- something I have to do regularly since I ride the bike daily in an urban environment (albeit without stuff like water bottles and air pumps on it).
mounting it on top of the top tube isn't possible because that's where the rear brake cable runs: The above-mentioned custom holder thing doesn't work there, and I can't simply use the lock holder's Velcro straps by themselves because I'm sure some wiseguy will steal the holder.


However, is the lock's positioning really so "strange"?-- its weight being so high and forward doesn't actually seem to affect handling nearly as much as I'd feared, which is the main reason I thought that people tend to mount stuff "low" and "back".

Lastly, I wrap my secondary, cable lock around the seat post because


I can't find a place on which to mount a proper holder for it.
the saddle bag featured in the photo of the previous bike doesn't actually fit on the newer bike's saddle.


In conclusion, the way I configured my bike's equipment was partially based on what I saw other bikes look like (e.g. with the bottle cage) and partially based on the quirks of my bike itself, which enforced constraints on where I "thought" accessories were supposed to go. Is this in fact what everyone does, and the second factor is what contributes to the variations in different riders' setups? Certainly, at least my "common sense" when it came to placing weight low and to the rear was actually a bit nonsensical.
For commuting with panniers and/or a toddler seat, I've settled on the front fork for the D-lock, mainly because little else fits there and I'm restricted by cables and tube diameters elsewhere. Having that 1.5kg forwards also helps the handling with a heavy back load. Don't forget that a heavy lock is only around 2% of a rider's weight - it has to be a long way from the centre of mass to be noticeable for handling. 

A cable lock will often fit in a spare bottle holder. I actually use the down tube holder for this, and the seat tube  one for my bottle. But I'm usually in stop-start conditions (for longer rides I use both bottle holders for bottles). 

My pump actually fits under the down tube with around 10mm clearance to the mudguard. 

My puncture kit (including for humans) and other tools go in a small triangular bag in the front of the frame. A saddle bag doesn't fit with the toddler seat. I'm lucky in that I normally lock up somewhere I'm happy to leave tools and lights on the bike. 

As for convention, if there is one I ignore it. Personal preference combined with quirks of bike design are much more important.