In a 100% speedrun, it is necessary that you collect as many quarters as possible throughout your run, because you need 807 coins in order to get all 40 jawbreakers. The game has a pre-set number of quarters in each scams, with some scams not having any. This is a guide that explains where all the quarters are located throughout the game.
Scam 1 - Cool Yer Ed
No quarters in this scam.
Scam 2 - Pin the Tail on the Ed
No quarters in this scam.
Scam 3 Pt. 1 - Must be Something I Ed
No quarters in this scam.
Scam 3 Pt. 2 - Must be Something I Ed
Quarter #1 - Next to Johnny, inside of the doghouse area where the jawbreaker in Part 1 originally was.
Bonus Scam 1 - Revenge of Ed-Zilla
No quarters in this scam.
Scam 4 - Ed on Arrival
Quarter #1 - Located between the switch that opens the gates to the swinging washing machine and the swinging washing machine itself. Batter-ed is needed to break the wall down and the quarter is located inside the crate.
Quarter #2 - Located behind the blue power cell platform inside of a crate.
Quarter #3 - Located inside of the rat pen of the red power cell inside of a crate.
Scam 5 - Nightmare on Ed Street
Quarter #1 - Located inside of the first wardrobe at the beginning of the level, needs to be broken with the Batter-Ed. (note: This quarter is farmed later on in the run because it is very fast; about ~40 seconds each trip on GCN.)
Quarter #2 - Located inside of the second wardrobe at the beginning of the axle grease segment. It is possible to hit this wardrobe through the wall with the Batter-Ed in order to save time, however, you need to be fast in order to grab the quarter as it will disappear if you don't grab it in time.
Quarter #3 - Located inside of the third wardrobe that covers a switch right after the Tunnel of Love section.
Scam 6 - Ed Marks the Spot
Quarter #1 - Located inside of the crates to the right, right after you open the power cell gate.
Quarter #2 - Located inside of a crate on a muddy island on the way to the Spider Egg. Double D's slingshot and Tower of Eddy are needed to reach it.
Quarters #3-7 - Victor's quarters; you fight Victor seven times in 100%; 5 quarters, 1 jawbreaker, 1 map piece.
Quarter #8 - Located in between the pipes in the sandbox/quadruple dog room of the Kanker Sister's tunnel; Trampol-Edd is needed to reach it.
Bonus Scam 2 - Rebel Robot Ranch
No quarters in this scam.
In total, there are 15 quarters in the game. This equates to a grand total of 375 coins (15¤25) obtained through just quarters alone. 807 coins minus 375 coins equals 432 coins that need to obtained through other means, ie. farming & collecting pennies throughout the run.
If there are any inaccuracies with this guide please let me know. Other versions of the game MIGHT have different numbers of quarters.
Fun fact: I didn't know that 'Quarter #2' in Scam 4 existed until I cut the 1 in 100%.