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 $56.97   3.303%   $58.8517   $58.85   $58.85   $58.90  
  Purchase #2 $41.76   8.349%   $45.2465   $45.25   $45.25   $45.20  
  Purchase #3 $111.88   8.394%   $121.2712   $121.27   $121.25   $121.30  
  Purchase #4 $19.89   10.686%   $22.0154   $22.02   $22.00   $22.00  
  Purchase #5 $122.25   12.746%   $137.832   $137.83   $137.85   $137.80  
  Purchase #6 $90.37   12.432%   $101.6048   $101.60   $101.60   $101.60  
  Purchase #7 $239.56   10.220%   $264.043   $264.04   $264.05   $264.00  
  7 purchase totals = $682.68       $750.8646   $750.86   $750.85   $750.80