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 $128.50   8.535%   $139.4675   $139.47   $139.45   $139.50  
  Purchase #2 $45.84   10.854%   $50.8155   $50.82   $50.80   $50.80  
  Purchase #3 $170.85   13.610%   $194.1027   $194.10   $194.10   $194.10  
  Purchase #4 $69.75   3.223%   $71.998   $72.00   $72.00   $72.00  
  Purchase #5 $232.98   6.172%   $247.3595   $247.36   $247.35   $247.40  
  Purchase #6 $73.72   7.667%   $79.3721   $79.37   $79.35   $79.40  
  Purchase #7 $118.79   13.380%   $134.6841   $134.68   $134.70   $134.70  
  7 purchase totals = $840.43       $917.7994   $917.80   $917.75   $917.90