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 $128.53   6.645%   $137.0708   $137.07   $137.05   $137.10  
  Purchase #2 $186.37   7.591%   $200.5173   $200.52   $200.50   $200.50  
  Purchase #3 $11.69   8.390%   $12.6708   $12.67   $12.65   $12.70  
  Purchase #4 $84.49   6.578%   $90.0478   $90.05   $90.05   $90.00  
  Purchase #5 $137.34   12.175%   $154.0611   $154.06   $154.05   $154.10  
  Purchase #6 $65.37   4.282%   $68.1691   $68.17   $68.15   $68.20  
  Purchase #7 $109.72   7.328%   $117.7603   $117.76   $117.75   $117.80  
  7 purchase totals = $723.51       $780.2972   $780.30   $780.20   $780.40