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 $28.87   12.848%   $32.5792   $32.58   $32.60   $32.60  
  Purchase #2 $225.73   7.289%   $242.1835   $242.18   $242.20   $242.20  
  Purchase #3 $51.67   9.716%   $56.6903   $56.69   $56.70   $56.70  
  Purchase #4 $138.67   9.263%   $151.515   $151.52   $151.50   $151.50  
  Purchase #5 $36.45   9.488%   $39.9084   $39.91   $39.90   $39.90  
  Purchase #6 $62.61   13.575%   $71.1093   $71.11   $71.10   $71.10  
  Purchase #7 $249.57   4.286%   $260.2666   $260.27   $260.25   $260.30  
  7 purchase totals = $793.57       $854.2523   $854.26   $854.25   $854.30