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 $19.79   9.829%   $21.7352   $21.74   $21.75   $21.70  
  Purchase #2 $26.50   5.529%   $27.9652   $27.97   $27.95   $28.00  
  Purchase #3 $161.56   4.381%   $168.6379   $168.64   $168.65   $168.60  
  Purchase #4 $108.64   3.260%   $112.1817   $112.18   $112.20   $112.20  
  Purchase #5 $59.89   5.533%   $63.2037   $63.20   $63.20   $63.20  
  Purchase #6 $222.37   12.573%   $250.3286   $250.33   $250.35   $250.30  
  Purchase #7 $127.68   3.788%   $132.5165   $132.52   $132.50   $132.50  
  7 purchase totals = $726.43       $776.5688   $776.58   $776.60   $776.50