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 $19.91   9.211%   $21.7439   $21.74   $21.75   $21.70  
  Purchase #2 $226.83   13.486%   $257.4203   $257.42   $257.40   $257.40  
  Purchase #3 $139.44   7.256%   $149.5578   $149.56   $149.55   $149.60  
  Purchase #4 $192.70   10.479%   $212.893   $212.89   $212.90   $212.90  
  Purchase #5 $28.56   6.138%   $30.313   $30.31   $30.30   $30.30  
  Purchase #6 $79.37   7.792%   $85.5545   $85.55   $85.55   $85.60  
  Purchase #7 $204.51   13.246%   $231.5994   $231.60   $231.60   $231.60  
  7 purchase totals = $891.32       $989.0819   $989.07   $989.05   $989.10