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 $28.30   6.716%   $30.2006   $30.20   $30.20   $30.20  
  Purchase #2 $98.40   10.186%   $108.423   $108.42   $108.40   $108.40  
  Purchase #3 $122.71   4.502%   $128.2344   $128.23   $128.25   $128.20  
  Purchase #4 $191.60   12.837%   $216.1957   $216.20   $216.20   $216.20  
  Purchase #5 $27.30   10.407%   $30.1411   $30.14   $30.15   $30.10  
  Purchase #6 $172.51   7.700%   $185.7933   $185.79   $185.80   $185.80  
  Purchase #7 $32.73   8.846%   $35.6253   $35.63   $35.65   $35.60  
  7 purchase totals = $673.55       $734.6134   $734.61   $734.65   $734.50