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 $221.66   12.521%   $249.414   $249.41   $249.40   $249.40  
  Purchase #2 $177.25   10.224%   $195.372   $195.37   $195.35   $195.40  
  Purchase #3 $164.49   7.351%   $176.5817   $176.58   $176.60   $176.60  
  Purchase #4 $83.52   4.721%   $87.463   $87.46   $87.45   $87.50  
  Purchase #5 $211.82   11.760%   $236.73   $236.73   $236.75   $236.70  
  Purchase #6 $243.70   11.455%   $271.6158   $271.62   $271.60   $271.60  
  Purchase #7 $62.33   10.863%   $69.1009   $69.10   $69.10   $69.10  
  7 purchase totals = $1,164.77       $1,286.2774   $1,286.27   $1,286.25   $1,286.30