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 $248.70   4.741%   $260.4909   $260.49   $260.50   $260.50  
  Purchase #2 $82.98   12.775%   $93.5807   $93.58   $93.60   $93.60  
  Purchase #3 $154.76   13.306%   $175.3524   $175.35   $175.35   $175.40  
  Purchase #4 $82.64   8.848%   $89.952   $89.95   $89.95   $90.00  
  Purchase #5 $208.26   9.288%   $227.6032   $227.60   $227.60   $227.60  
  Purchase #6 $164.58   5.818%   $174.1553   $174.16   $174.15   $174.20  
  Purchase #7 $204.31   10.554%   $225.8729   $225.87   $225.85   $225.90  
  7 purchase totals = $1,146.23       $1,247.0074   $1,247.00   $1,247.00   $1,247.20