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 $181.68   13.675%   $206.5247   $206.52   $206.50   $206.50  
  Purchase #2 $144.87   10.589%   $160.2103   $160.21   $160.20   $160.20  
  Purchase #3 $58.64   12.358%   $65.8867   $65.89   $65.90   $65.90  
  Purchase #4 $179.63   12.872%   $202.752   $202.75   $202.75   $202.80  
  Purchase #5 $236.96   4.189%   $246.8863   $246.89   $246.90   $246.90  
  Purchase #6 $79.41   11.248%   $88.342   $88.34   $88.35   $88.30  
  Purchase #7 $17.96   10.713%   $19.8841   $19.88   $19.90   $19.90  
  7 purchase totals = $899.15       $990.4861   $990.48   $990.50   $990.50