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 $28.39   8.534%   $30.8128   $30.81   $30.80   $30.80  
  Purchase #2 $102.83   5.249%   $108.2275   $108.23   $108.25   $108.20  
  Purchase #3 $165.98   7.862%   $179.0293   $179.03   $179.05   $179.00  
  Purchase #4 $199.27   7.205%   $213.6274   $213.63   $213.65   $213.60  
  Purchase #5 $79.31   7.488%   $85.2487   $85.25   $85.25   $85.20  
  Purchase #6 $228.32   12.173%   $256.1134   $256.11   $256.10   $256.10  
  Purchase #7 $63.73   10.851%   $70.6453   $70.65   $70.65   $70.60  
  7 purchase totals = $867.83       $943.7044   $943.71   $943.75   $943.50