The nickel Keeps the screen clear by moving around on it. It helps the trichs fall through the screen
I use a british pound, smaller and heavier. might want to try that, depending on the size of the grinder
Would a nickel be the best american coin? Dime is smaller. Could move more? I guess its not as heavy.
If you have a 4 piece grinder you can put any coin in the screen/stash compartment but not where the actual keif collects, when you grind your herb it will fall into the chamber that the coin is in and if you shake it a bit the coin will move around knocking a lot of tricnomes off the bud therefor creating alot more keif. It works I do it all the time you'll be swimming in keif in no time.
I personally use a dime since my grinder isn't exsctly huge but it does defientely help collect kief its just short of having a kief box