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 $12.32   8.600%   $13.3795   $13.38   $13.40   $13.40  
  Purchase #2 $28.41   8.210%   $30.7425   $30.74   $30.75   $30.70  
  Purchase #3 $178.78   3.420%   $184.8943   $184.89   $184.90   $184.90  
  Purchase #4 $131.43   13.408%   $149.0521   $149.05   $149.05   $149.10  
  Purchase #5 $111.46   3.754%   $115.6442   $115.64   $115.65   $115.60  
  Purchase #6 $89.92   8.764%   $97.8006   $97.80   $97.80   $97.80  
  Purchase #7 $232.53   12.404%   $261.373   $261.37   $261.35   $261.40  
  7 purchase totals = $784.85       $852.8862   $852.87   $852.90   $852.90