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 $19.99   5.381%   $21.0657   $21.07   $21.05   $21.10  
  Purchase #2 $31.93   6.829%   $34.1105   $34.11   $34.10   $34.10  
  Purchase #3 $117.84   3.636%   $122.1247   $122.12   $122.10   $122.10  
  Purchase #4 $116.79   6.181%   $124.0088   $124.01   $124.00   $124.00  
  Purchase #5 $91.70   8.162%   $99.1846   $99.18   $99.20   $99.20  
  Purchase #6 $116.88   5.476%   $123.2803   $123.28   $123.30   $123.30  
  Purchase #7 $157.63   12.105%   $176.7111   $176.71   $176.70   $176.70  
  7 purchase totals = $652.76       $700.4857   $700.48   $700.45   $700.50