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 $20.77   4.205%   $21.6434   $21.64   $21.65   $21.60  
  Purchase #2 $18.74   8.387%   $20.3117   $20.31   $20.30   $20.30  
  Purchase #3 $183.85   5.306%   $193.6051   $193.61   $193.60   $193.60  
  Purchase #4 $154.61   13.515%   $175.5055   $175.51   $175.50   $175.50  
  Purchase #5 $23.94   6.460%   $25.4865   $25.49   $25.50   $25.50  
  Purchase #6 $13.57   7.596%   $14.6008   $14.60   $14.60   $14.60  
  Purchase #7 $137.39   8.264%   $148.7439   $148.74   $148.75   $148.70  
  7 purchase totals = $552.87       $599.8969   $599.90   $599.90   $599.80