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 $19.98   5.431%   $21.0651   $21.07   $21.05   $21.10  
  Purchase #2 $27.69   7.804%   $29.8509   $29.85   $29.85   $29.90  
  Purchase #3 $38.61   6.545%   $41.137   $41.14   $41.15   $41.10  
  Purchase #4 $150.97   8.532%   $163.8508   $163.85   $163.85   $163.90  
  Purchase #5 $73.80   13.156%   $83.5091   $83.51   $83.50   $83.50  
  Purchase #6 $134.42   6.323%   $142.9194   $142.92   $142.90   $142.90  
  Purchase #7 $24.49   12.786%   $27.6213   $27.62   $27.60   $27.60  
  7 purchase totals = $469.96       $509.9536   $509.96   $509.90   $510.00