The closer you are to the car in front of you, the faster they will start up and go when the light turns green. I don’t know how this works, but it’s the same as with an elevator button, the more times you push it, the faster the elevator comes. Some things were just never meant to be understood.
I’m ashamed to say that I’m a chronic tailgater! I’ve unintentionally angered so many people I’ve been behind in a car that they’ve actually pulled over to let me pass. I honestly don’t mean to do it but the next thing I know, I’m riding someone’s a$$. I think it stems from my lifelong habit of being in a hurry to get to wherever I need to go, whether walking or driving.
I apologize to everyone out there for all the tailgaters like me. I hate it when someone does it to me so I’ve really been making a huge effort to stop doing it to others. It’s totally unintentional on my part, I promise! 
5 - 10 feet I guess. I don’t always leave enough room to be able to see the bottom of the rear wheels but always enough to be able to change lanes without reversing in case the car in front can’t move. Being rear ended seems rare enough that it doesn’t enter into my thinking.
Around a couple of feet, especially in heavy traffic when there are a lot of cars behind me. It makes the line shorter so people in cross-streets can get through.
Except when the car in front of me is producing exhaust fumes.
That’s what I was taught. In the winter I add about 5’.
Ummmmm huh? Where else would one drive? It’s not all dirt roads in Alaska you know. 
Heh! That’s okay, picture ye olde grandma sitting in her living room, watching the Back Street Boys (didn’t they grow up and become not a boy band anymore a long time ago?) and the NYC ball drop thing, quizzically staring at her computer wracking her brain for the word she was supposed to have used.
Took note of what I actually do tonight and I see the bottom of the wheels plus some road.
Well, technically, it’s the hard surface, not the type of way. “The road is paved with asphalt. The sidewalk is paved with concrete. The two pavements differ.”
The first stick I owned didn’t have a handbrake. It had the stupid 4th pedal parking brake that was difficult to use to get started. So I learned to engage the clutch before taking my foot off the brake. You let the clutch out until you feel it start to grab, then you take your foot off the brake and hit the accelerator.
It should only happen for a split second and won’t put any more wear on the clutch than normal start and stop driving.
I should note that I live in southeastern Louisiana, where there are NO hills. The only time I ever really had to employ this technique was at a nearby intersection that has a rise approaching railroad tracks, and occasionally on an on-ramp or bridge in heavy traffic.
ETA: What I learned in driver’s ed was to leave an escape route. The car in front of you breaks down, you don’t want to have to back up. The rule of thumb was to stop while you can still see the tires on the pavement, which can range from 3 feet to 12 feet depending on whether you have a really long hood or not.
Realistically, you only need about 2-3 feet to turn the front wheels and get out of your lane without backing. If it took more than that, parallel parking would be a nightmare.
Depends:
-I will usually pull up tight if next to a turn lane, so as to allow as much access to the turn lane as possible. IMO this is the only time pulling up tight is good form.
-I won’t usually pull up if the cars ahead of me are edging forward. It just seems dumb when nobody is going anywhere until the light turns green anyway. Often this will leave an extra car length or even more for a while.
-In light traffic, I will usually slow early so as to not need to stop at all.
-One of my vehicles is pretty tall. If I pull up close to a small car at night, my headlights are mighty bright. I will usually stay a bit farther back, but if there is a reason to pull up, I will switch off the headlights until the light changes.
-I won’t block intersections, and I may pull up tight or hang back as needed to allow someone to get out of a driveway.
-When on a motorcyle, I try to be where people behind would expect to find me, but also back far enough that I can split the lanes if my mirror indicates the car approaching behind me may not stop in time. Only had to actually do that once.
I have been rear-ended twice while waiting at a light. Being a bit farther back meant there was no damage to the front of my vehicle, and no issue with insurance claims or tickets, since my vehicle didn’t rear-end anyone.
I also wrote five to ten feet. I drive a smart car and it FREAKS me out when I feel like I can reach back through the rear window and touch the front bumper of the car behind me. Stop steaming up my tail!!!
What I see more and more often, and maybe this is just with an older generation, is when cars leave upwards of 15 feet in between them and the car in front of them. What the hell is the point of that? Double points if you are the first car in the line, and you leave 15 feet between you and the line. Especially if the light has a weight sensor that will not change the light until it detects a vehicle. I always want to yell at people I see doing this because it is a complete waste of space and has no purpose.
Ideally, I leave enough space to get out of there in an emergency. Enough so my turn radius will take me into the other lane (or over the sidewalk :D), without bumping the car ahead. I’m guessing that amounts to about 5 to 10 feet, but I checked “Depends”, instead, because it’s a judgement call based on what else is in the way, so it could be longer.