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 $152.43   13.639%   $173.2199   $173.22   $173.20   $173.20  
  Purchase #2 $69.63   12.494%   $78.3296   $78.33   $78.35   $78.30  
  Purchase #3 $196.51   13.583%   $223.202   $223.20   $223.20   $223.20  
  Purchase #4 $233.96   9.735%   $256.736   $256.74   $256.75   $256.70  
  Purchase #5 $125.25   5.822%   $132.5421   $132.54   $132.55   $132.50  
  Purchase #6 $63.26   10.647%   $69.9953   $70.00   $70.00   $70.00  
  Purchase #7 $26.27   8.107%   $28.3997   $28.40   $28.40   $28.40  
  7 purchase totals = $867.31       $962.4246   $962.43   $962.45   $962.30