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 $139.33   3.655%   $144.4225   $144.42   $144.40   $144.40  
  Purchase #2 $97.91   5.379%   $103.1766   $103.18   $103.20   $103.20  
  Purchase #3 $9.97   8.490%   $10.8165   $10.82   $10.80   $10.80  
  Purchase #4 $12.49   7.711%   $13.4531   $13.45   $13.45   $13.50  
  Purchase #5 $151.32   12.218%   $169.8083   $169.81   $169.80   $169.80  
  Purchase #6 $36.77   9.310%   $40.1933   $40.19   $40.20   $40.20  
  Purchase #7 $50.96   8.147%   $55.1117   $55.11   $55.10   $55.10  
  7 purchase totals = $498.75       $536.9820   $536.98   $536.95   $537.00