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 $49.31   8.451%   $53.4772   $53.48   $53.50   $53.50  
  Purchase #2 $66.60   8.475%   $72.2444   $72.24   $72.25   $72.20  
  Purchase #3 $234.85   12.838%   $265   $265.00   $265.00   $265.00  
  Purchase #4 $225.25   4.616%   $235.6475   $235.65   $235.65   $235.60  
  Purchase #5 $116.59   13.276%   $132.0685   $132.07   $132.05   $132.10  
  Purchase #6 $55.89   4.693%   $58.5129   $58.51   $58.50   $58.50  
  Purchase #7 $137.26   3.531%   $142.1067   $142.11   $142.10   $142.10  
  7 purchase totals = $885.75       $959.0572   $959.06   $959.05   $959.00