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 $27.31   5.417%   $28.7894   $28.79   $28.80   $28.80  
  Purchase #2 $172.66   6.646%   $184.135   $184.14   $184.15   $184.10  
  Purchase #3 $63.88   4.838%   $66.9705   $66.97   $66.95   $67.00  
  Purchase #4 $15.36   3.218%   $15.8543   $15.85   $15.85   $15.90  
  Purchase #5 $76.40   8.775%   $83.1041   $83.10   $83.10   $83.10  
  Purchase #6 $82.87   8.271%   $89.7242   $89.72   $89.70   $89.70  
  Purchase #7 $163.55   11.477%   $182.3206   $182.32   $182.30   $182.30  
  7 purchase totals = $602.03       $650.8981   $650.89   $650.85   $650.90