You are NOT REQUIRED to remove your shoes before you enter the walk-through metal detector.  However, TSA screeners may encourage you to remove them before entering the metal detector as many types of footwear will require additional screening even if the metal detector DOES NOT alarm.  

Footwear that screeners will encourage you to remove because they are likely to require additional screening:

Boots  
Platform shoes (including platform flip-flops)
Footwear with a thick sole or heel (including athletic shoes)
Footwear containing metal (including many dress shoes)

Footwear that screeners are less likely to suggest you remove includes:

"Beach" flip-flops
Thin-soled sandals (without metal)

TIP: Since a thorough screening includes x-ray inspection of footwear, wearing footwear that is easily removable helps to speed you through the screening process.