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 $26.40   6.425%   $28.0962   $28.10   $28.10   $28.10  
  Purchase #2 $19.75   6.206%   $20.9757   $20.98   $21.00   $21.00  
  Purchase #3 $198.87   5.335%   $209.4797   $209.48   $209.50   $209.50  
  Purchase #4 $231.40   7.607%   $249.0026   $249.00   $249.00   $249.00  
  Purchase #5 $163.76   11.208%   $182.1142   $182.11   $182.10   $182.10  
  Purchase #6 $84.58   9.631%   $92.7259   $92.73   $92.75   $92.70  
  Purchase #7 $173.70   8.467%   $188.4072   $188.41   $188.40   $188.40  
  7 purchase totals = $898.46       $970.8015   $970.81   $970.85   $970.80