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 $17.67   5.300%   $18.6065   $18.61   $18.60   $18.60  
  Purchase #2 $17.31   13.503%   $19.6474   $19.65   $19.65   $19.60  
  Purchase #3 $132.42   9.740%   $145.3177   $145.32   $145.30   $145.30  
  Purchase #4 $76.62   4.202%   $79.8396   $79.84   $79.85   $79.80  
  Purchase #5 $27.65   4.511%   $28.8973   $28.90   $28.90   $28.90  
  Purchase #6 $18.81   11.536%   $20.9799   $20.98   $21.00   $21.00  
  Purchase #7 $129.69   3.675%   $134.4561   $134.46   $134.45   $134.50  
  7 purchase totals = $420.17       $447.7445   $447.76   $447.75   $447.70