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 $13.65   10.299%   $15.0558   $15.06   $15.05   $15.10  
  Purchase #2 $77.28   10.344%   $85.2738   $85.27   $85.25   $85.30  
  Purchase #3 $40.84   10.264%   $45.0318   $45.03   $45.05   $45.00  
  Purchase #4 $161.90   4.396%   $169.0171   $169.02   $169.00   $169.00  
  Purchase #5 $47.92   5.594%   $50.6006   $50.60   $50.60   $50.60  
  Purchase #6 $145.37   10.115%   $160.0742   $160.07   $160.05   $160.10  
  Purchase #7 $24.99   8.435%   $27.0979   $27.10   $27.10   $27.10  
  7 purchase totals = $511.95       $552.1512   $552.15   $552.10   $552.20