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 $51.77   13.479%   $58.7481   $58.75   $58.75   $58.70  
  Purchase #2 $150.89   9.105%   $164.6285   $164.63   $164.65   $164.60  
  Purchase #3 $132.68   12.273%   $148.9638   $148.96   $148.95   $149.00  
  Purchase #4 $98.68   13.260%   $111.765   $111.77   $111.75   $111.80  
  Purchase #5 $24.78   4.667%   $25.9365   $25.94   $25.95   $25.90  
  Purchase #6 $246.66   10.372%   $272.2436   $272.24   $272.25   $272.20  
  Purchase #7 $56.63   3.298%   $58.4977   $58.50   $58.50   $58.50  
  7 purchase totals = $762.09       $840.7832   $840.79   $840.80   $840.70