For me, I put 
on F because I played without it until 
's launch, since I didn't try using it ever. However, jungling was my favorite role, and always put 
on D since I'd very rarely swap off of 
(which is put on F when you first make your account). D has always been my key, so as a result, F ended up becoming my key.
What key do you put your flash on, and do you have any reason why you picked that key?
D and I absolutely never change it. Been using it like this since I started playing the game around 5 years ago
D
It's faster than F.
I have it on f. The reason for that is having the f key being the farther of the two, it taught me not to panic smack the key when there were situations I couldn’t think my way out of instead of using flash.
Plus when I do need it last second my brain can easily think press f for flash.
d= ignite f= smite
Shaco One trick forever
Flash goes on F and only F. People that put it on D have flat-earth level insanity.
D for death F for Flash
I put it on D because it's naturally where my index finger is, so I can click it as fast as possible. Every other summoner for me requires a bit more deliberation (usually heal or ignite) and I can hover down to F for that. Flash needs that split second reaction timing, both for offense and defense, and that 0.1 seconds it takes for my finger to move half an inch could mean life or death, or kill/no kill.
F for flash D for don't put flash here
D is for dash, which rhymes with Flash.
F is for fire 
Flash = C
Other summ = V
I've got big hands so I end up using my thumb, also because previously I was playing WoW 
F for flash D for definitely anything but flash.
flash is for noobs!
I have on D because I started playing like this from the beginning.
F=
D= 
is unbound
probably also depends on if you play with 4 or 3 fingers
on D because you need the key to be closer to your finger for faster reaction time being agressive or defensive flash.
D F does it matter?
D because I have the force of habit of my fingers on the WAD buttons (shooter games), as well as D being faster in that respect. Once you get the habit of not panic flashing, using it quickly is more important and not costly. Also applies to Ult (R) being a tactical decision that takes longer than putting my finger on it (with most champs I use anyways).
I put summs on the side mouse buttons. Upper one for flash.
I play with all 5 fingers.
My thumb is responsible for the spacebar key, and the D + F key.
I put flash on D because it's almost impossible for me to accidentally flash without reaching for it. F however is just close enough for my thumb to touch first increasing my chances of accidentally clicking it when I do go for the D key.
Mine flash is on F in lane and D in jungle.
I do this because I'm color blind, so smite and flash look like the same thing, icon wise.
Flash on D cuz flash on f is for losers.
There's only one exception and that's singed in aram, cuz ghost on D takes priority over Flash on D and snowball is thrash so flash becomes a "regular" summoner that isn't mandatory and thus is allowed on the F key.
D is for 'D'flash', F is 'For Everything else'.
Honestly, it's the setup I chose and it's stayed that way. My first game of Blitz, it set it the other way around when I was assigned jungle, I flashed early not realising.
D. In most games I usually don't have ti extend to my F key except as an auxiliary chat key. D feels comfy for me and F is for everything else I will seldom use.
D is flash and G Ignite if I put F on ignite i sometimes missclick
On D because it is easier to reach and I try to only flash when I absolutely need to.
D because if I end up panicking im more likely to hit my flash key. I like the summoner I can rely on to be closest to me.
x, I use wasd for camera controls and I have abilities maped to my mouse. I also have actives mapped to zxcvb and e.
F. Not because F is for flash but because its the most valuable summoner spell, and the key is right below my ult key
I put Smite on D whenever I jungled but the rest of the time I would put Flash on D and my other summoner spell on F. I finally forced myself to put Smite on F and Flash on D to keep myself from fail-flashing when jungling or accidentally teleporting to the tower next to me when trying to flash away from a gank.
mine's always the D key. it's too easy to flash accidentally when you're trying to hit a ping key
though flash is not one my favorite summoner spells anyway
my summoner spells are bound to F1 and F2. i do that so my W and my E can reach them without too much trouble.
