Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $86.37   4.306%   $90.0891   $90.09   $90.10   $90.10  
  Purchase #2 $132.34   12.145%   $148.4127   $148.41   $148.40   $148.40  
  Purchase #3 $139.49   3.392%   $144.2215   $144.22   $144.20   $144.20  
  Purchase #4 $29.66   5.816%   $31.385   $31.39   $31.40   $31.40  
  Purchase #5 $165.44   3.193%   $170.7225   $170.72   $170.70   $170.70  
  Purchase #6 $115.55   10.772%   $127.997   $128.00   $128.00   $128.00  
  Purchase #7 $103.29   3.755%   $107.1685   $107.17   $107.15   $107.20  
  7 purchase totals = $772.14       $819.9963   $820.00   $819.95   $820.00