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 $186.47   12.349%   $209.4972   $209.50   $209.50   $209.50  
  Purchase #2 $134.63   5.515%   $142.0548   $142.05   $142.05   $142.10  
  Purchase #3 $182.62   6.204%   $193.9497   $193.95   $193.95   $193.90  
  Purchase #4 $76.46   12.821%   $86.2629   $86.26   $86.25   $86.30  
  Purchase #5 $226.76   10.154%   $249.7852   $249.79   $249.80   $249.80  
  Purchase #6 $27.73   6.512%   $29.5358   $29.54   $29.55   $29.50  
  Purchase #7 $73.82   11.313%   $82.1713   $82.17   $82.15   $82.20  
  7 purchase totals = $908.49       $993.2569   $993.26   $993.25   $993.30