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 $239.65   7.511%   $257.6501   $257.65   $257.65   $257.70  
  Purchase #2 $16.59   8.680%   $18.03   $18.03   $18.05   $18.00  
  Purchase #3 $85.67   4.106%   $89.1876   $89.19   $89.20   $89.20  
  Purchase #4 $231.74   7.709%   $249.6048   $249.60   $249.60   $249.60  
  Purchase #5 $75.26   12.549%   $84.7044   $84.70   $84.70   $84.70  
  Purchase #6 $63.63   12.146%   $71.3585   $71.36   $71.35   $71.40  
  Purchase #7 $24.39   9.837%   $26.7892   $26.79   $26.80   $26.80  
  7 purchase totals = $736.93       $797.3246   $797.32   $797.35   $797.40