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 $240.66   13.525%   $273.2093   $273.21   $273.20   $273.20  
  Purchase #2 $182.28   4.568%   $190.6066   $190.61   $190.60   $190.60  
  Purchase #3 $161.68   4.767%   $169.3873   $169.39   $169.40   $169.40  
  Purchase #4 $246.89   5.263%   $259.8838   $259.88   $259.90   $259.90  
  Purchase #5 $57.73   7.422%   $62.0147   $62.01   $62.00   $62.00  
  Purchase #6 $62.48   13.218%   $70.7386   $70.74   $70.75   $70.70  
  Purchase #7 $12.91   8.147%   $13.9618   $13.96   $13.95   $14.00  
  7 purchase totals = $964.63       $1,039.8021   $1,039.80   $1,039.80   $1,039.80