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 $88.47   11.127%   $98.3141   $98.31   $98.30   $98.30  
  Purchase #2 $70.78   7.388%   $76.0092   $76.01   $76.00   $76.00  
  Purchase #3 $146.69   13.607%   $166.6501   $166.65   $166.65   $166.70  
  Purchase #4 $35.92   13.686%   $40.836   $40.84   $40.85   $40.80  
  Purchase #5 $6.47   4.504%   $6.7614   $6.76   $6.75   $6.80  
  Purchase #6 $62.97   9.574%   $68.9987   $69.00   $69.00   $69.00  
  Purchase #7 $132.94   4.254%   $138.5953   $138.60   $138.60   $138.60  
  7 purchase totals = $544.24       $596.1648   $596.17   $596.15   $596.20