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 $28.46   5.255%   $29.9556   $29.96   $29.95   $30.00  
  Purchase #2 $9.83   9.291%   $10.7433   $10.74   $10.75   $10.70  
  Purchase #3 $113.90   13.367%   $129.125   $129.13   $129.15   $129.10  
  Purchase #4 $158.32   4.388%   $165.2671   $165.27   $165.25   $165.30  
  Purchase #5 $189.31   12.608%   $213.1782   $213.18   $213.20   $213.20  
  Purchase #6 $44.81   7.138%   $48.0085   $48.01   $48.00   $48.00  
  Purchase #7 $7.26   8.195%   $7.855   $7.86   $7.85   $7.90  
  7 purchase totals = $551.89       $604.1327   $604.15   $604.15   $604.20