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 $19.84   8.325%   $21.4917   $21.49   $21.50   $21.50  
  Purchase #2 $83.96   11.479%   $93.5978   $93.60   $93.60   $93.60  
  Purchase #3 $54.61   13.330%   $61.8895   $61.89   $61.90   $61.90  
  Purchase #4 $185.63   7.130%   $198.8654   $198.87   $198.85   $198.90  
  Purchase #5 $107.62   9.408%   $117.7449   $117.74   $117.75   $117.70  
  Purchase #6 $90.94   6.365%   $96.7283   $96.73   $96.75   $96.70  
  Purchase #7 $46.80   12.471%   $52.6364   $52.64   $52.65   $52.60  
  7 purchase totals = $589.40       $642.9540   $642.96   $643.00   $642.90