Buddy, can you spare a dime?
The organization known as the March of Dimes started out as the National Foundation for Infantile Paralysis, whose fundraising capabilities were greatly enhanced by FDR's personal interest and involvement. His Presidential Birthday Parties, devoted to raising money to combat polio, formed the basis for what became known as the March of Dimes.
This is the reason the dime was chosen for FDR. He earned that dime, in more ways than one. He keeps it.