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 $17.81   3.314%   $18.4002   $18.40   $18.40   $18.40  
  Purchase #2 $158.31   12.743%   $178.4834   $178.48   $178.50   $178.50  
  Purchase #3 $192.88   3.286%   $199.218   $199.22   $199.20   $199.20  
  Purchase #4 $159.43   4.172%   $166.0814   $166.08   $166.10   $166.10  
  Purchase #5 $124.57   9.855%   $136.8464   $136.85   $136.85   $136.80  
  Purchase #6 $21.81   6.563%   $23.2414   $23.24   $23.25   $23.20  
  Purchase #7 $149.71   10.624%   $165.6152   $165.62   $165.60   $165.60  
  7 purchase totals = $824.52       $887.8860   $887.89   $887.90   $887.80