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 $20.78   6.599%   $22.1513   $22.15   $22.15   $22.20  
  Purchase #2 $27.31   7.724%   $29.4194   $29.42   $29.40   $29.40  
  Purchase #3 $92.62   11.435%   $103.2111   $103.21   $103.20   $103.20  
  Purchase #4 $92.52   6.189%   $98.2461   $98.25   $98.25   $98.20  
  Purchase #5 $66.83   3.540%   $69.1958   $69.20   $69.20   $69.20  
  Purchase #6 $21.62   13.343%   $24.5048   $24.50   $24.50   $24.50  
  Purchase #7 $144.62   10.549%   $159.876   $159.88   $159.90   $159.90  
  7 purchase totals = $466.30       $506.6045   $506.61   $506.60   $506.60