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 $9.50   4.709%   $9.9474   $9.95   $9.95   $9.90  
  Purchase #2 $10.66   13.707%   $12.1212   $12.12   $12.10   $12.10  
  Purchase #3 $87.35   5.874%   $92.4809   $92.48   $92.50   $92.50  
  Purchase #4 $93.26   8.642%   $101.3195   $101.32   $101.30   $101.30  
  Purchase #5 $105.85   13.521%   $120.162   $120.16   $120.15   $120.20  
  Purchase #6 $141.89   4.461%   $148.2197   $148.22   $148.20   $148.20  
  Purchase #7 $106.63   6.211%   $113.2528   $113.25   $113.25   $113.30  
  7 purchase totals = $555.14       $597.5035   $597.50   $597.45   $597.50