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 $18.92   11.682%   $21.1302   $21.13   $21.15   $21.10  
  Purchase #2 $77.44   9.158%   $84.532   $84.53   $84.55   $84.50  
  Purchase #3 $125.86   8.741%   $136.8614   $136.86   $136.85   $136.90  
  Purchase #4 $92.31   13.729%   $104.9832   $104.98   $105.00   $105.00  
  Purchase #5 $123.26   10.487%   $136.1863   $136.19   $136.20   $136.20  
  Purchase #6 $77.68   4.196%   $80.9395   $80.94   $80.95   $80.90  
  Purchase #7 $18.71   4.606%   $19.5718   $19.57   $19.55   $19.60  
  7 purchase totals = $534.18       $584.2044   $584.20   $584.25   $584.20