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 $9.75   4.319%   $10.1711   $10.17   $10.15   $10.20  
  Purchase #2 $36.83   6.641%   $39.2759   $39.28   $39.30   $39.30  
  Purchase #3 $42.28   3.420%   $43.726   $43.73   $43.75   $43.70  
  Purchase #4 $109.27   11.654%   $122.0043   $122.00   $122.00   $122.00  
  Purchase #5 $12.50   8.651%   $13.5814   $13.58   $13.60   $13.60  
  Purchase #6 $234.89   12.518%   $264.2935   $264.29   $264.30   $264.30  
  Purchase #7 $42.53   13.820%   $48.4076   $48.41   $48.40   $48.40  
  7 purchase totals = $488.05       $541.4598   $541.46   $541.50   $541.50