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 $28.89   4.230%   $30.112   $30.11   $30.10   $30.10  
  Purchase #2 $217.46   10.560%   $240.4238   $240.42   $240.40   $240.40  
  Purchase #3 $178.27   10.363%   $196.7441   $196.74   $196.75   $196.70  
  Purchase #4 $5.64   8.239%   $6.1047   $6.10   $6.10   $6.10  
  Purchase #5 $120.90   6.802%   $129.1236   $129.12   $129.10   $129.10  
  Purchase #6 $35.53   8.682%   $38.6147   $38.61   $38.60   $38.60  
  Purchase #7 $132.35   7.571%   $142.3702   $142.37   $142.35   $142.40  
  7 purchase totals = $719.04       $783.4931   $783.47   $783.40   $783.40