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 $26.43   11.342%   $29.4277   $29.43   $29.45   $29.40  
  Purchase #2 $64.36   3.390%   $66.5418   $66.54   $66.55   $66.50  
  Purchase #3 $186.47   7.127%   $199.7597   $199.76   $199.75   $199.80  
  Purchase #4 $116.48   6.516%   $124.0698   $124.07   $124.05   $124.10  
  Purchase #5 $93.66   5.560%   $98.8675   $98.87   $98.85   $98.90  
  Purchase #6 $250.72   10.403%   $276.8024   $276.80   $276.80   $276.80  
  Purchase #7 $163.75   7.655%   $176.2851   $176.29   $176.30   $176.30  
  7 purchase totals = $901.87       $971.7540   $971.76   $971.75   $971.80