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 $242.75   4.141%   $252.8023   $252.80   $252.80   $252.80  
  Purchase #2 $78.83   10.239%   $86.9014   $86.90   $86.90   $86.90  
  Purchase #3 $24.74   10.237%   $27.2726   $27.27   $27.25   $27.30  
  Purchase #4 $243.54   6.819%   $260.147   $260.15   $260.15   $260.10  
  Purchase #5 $157.35   13.599%   $178.748   $178.75   $178.75   $178.70  
  Purchase #6 $102.76   12.504%   $115.6091   $115.61   $115.60   $115.60  
  Purchase #7 $76.80   5.359%   $80.9157   $80.92   $80.90   $80.90  
  7 purchase totals = $926.77       $1,002.3961   $1,002.40   $1,002.35   $1,002.30