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 $45.45   8.641%   $49.3773   $49.38   $49.40   $49.40  
  Purchase #2 $20.85   12.811%   $23.5211   $23.52   $23.50   $23.50  
  Purchase #3 $125.78   12.388%   $141.3616   $141.36   $141.35   $141.40  
  Purchase #4 $122.42   12.411%   $137.6135   $137.61   $137.60   $137.60  
  Purchase #5 $68.98   11.553%   $76.9493   $76.95   $76.95   $76.90  
  Purchase #6 $19.54   10.296%   $21.5518   $21.55   $21.55   $21.60  
  Purchase #7 $85.48   8.418%   $92.6757   $92.68   $92.70   $92.70  
  7 purchase totals = $488.50       $543.0503   $543.05   $543.05   $543.10