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 $27.33   4.448%   $28.5456   $28.55   $28.55   $28.50  
  Purchase #2 $95.49   10.348%   $105.3713   $105.37   $105.35   $105.40  
  Purchase #3 $84.25   12.542%   $94.8166   $94.82   $94.80   $94.80  
  Purchase #4 $121.81   7.365%   $130.7813   $130.78   $130.80   $130.80  
  Purchase #5 $7.46   13.850%   $8.4932   $8.49   $8.50   $8.50  
  Purchase #6 $116.85   13.683%   $132.8386   $132.84   $132.85   $132.80  
  Purchase #7 $141.80   11.123%   $157.5724   $157.57   $157.55   $157.60  
  7 purchase totals = $594.99       $658.4190   $658.42   $658.40   $658.40