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 $17.38   7.800%   $18.7356   $18.74   $18.75   $18.70  
  Purchase #2 $5.32   8.391%   $5.7664   $5.77   $5.75   $5.80  
  Purchase #3 $241.27   12.635%   $271.7545   $271.75   $271.75   $271.80  
  Purchase #4 $17.71   13.785%   $20.1513   $20.15   $20.15   $20.20  
  Purchase #5 $188.96   3.127%   $194.8688   $194.87   $194.85   $194.90  
  Purchase #6 $77.81   4.352%   $81.1963   $81.20   $81.20   $81.20  
  Purchase #7 $86.85   12.622%   $97.8122   $97.81   $97.80   $97.80  
  7 purchase totals = $635.30       $690.2851   $690.29   $690.25   $690.40