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 $19.78   6.549%   $21.0754   $21.08   $21.10   $21.10  
  Purchase #2 $123.88   10.485%   $136.8688   $136.87   $136.85   $136.90  
  Purchase #3 $105.83   5.604%   $111.7607   $111.76   $111.75   $111.80  
  Purchase #4 $157.56   10.278%   $173.754   $173.75   $173.75   $173.80  
  Purchase #5 $235.44   9.232%   $257.1758   $257.18   $257.20   $257.20  
  Purchase #6 $80.34   5.727%   $84.9411   $84.94   $84.95   $84.90  
  Purchase #7 $19.32   11.607%   $21.5625   $21.56   $21.55   $21.60  
  7 purchase totals = $742.15       $807.1383   $807.14   $807.15   $807.30