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 $21.75   6.342%   $23.1294   $23.13   $23.15   $23.10  
  Purchase #2 $178.64   8.796%   $194.3532   $194.35   $194.35   $194.40  
  Purchase #3 $27.97   9.398%   $30.5986   $30.60   $30.60   $30.60  
  Purchase #4 $7.53   5.555%   $7.9483   $7.95   $7.95   $7.90  
  Purchase #5 $145.52   7.302%   $156.1459   $156.15   $156.15   $156.10  
  Purchase #6 $246.69   13.478%   $279.9389   $279.94   $279.95   $279.90  
  Purchase #7 $17.50   11.102%   $19.4429   $19.44   $19.45   $19.40  
  7 purchase totals = $645.60       $711.5572   $711.56   $711.60   $711.40