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 $20.91   3.612%   $21.6653   $21.67   $21.65   $21.70  
  Purchase #2 $227.80   8.741%   $247.712   $247.71   $247.70   $247.70  
  Purchase #3 $137.60   10.720%   $152.3507   $152.35   $152.35   $152.40  
  Purchase #4 $114.68   13.655%   $130.3396   $130.34   $130.35   $130.30  
  Purchase #5 $82.99   13.394%   $94.1057   $94.11   $94.10   $94.10  
  Purchase #6 $78.69   8.606%   $85.4621   $85.46   $85.45   $85.50  
  Purchase #7 $224.33   5.829%   $237.4062   $237.41   $237.40   $237.40  
  7 purchase totals = $887.00       $969.0416   $969.05   $969.00   $969.10