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 $122.30   6.192%   $129.8728   $129.87   $129.85   $129.90  
  Purchase #2 $148.30   6.360%   $157.7319   $157.73   $157.75   $157.70  
  Purchase #3 $214.41   11.768%   $239.6418   $239.64   $239.65   $239.60  
  Purchase #4 $92.26   10.383%   $101.8394   $101.84   $101.85   $101.80  
  Purchase #5 $8.74   4.772%   $9.1571   $9.16   $9.15   $9.20  
  Purchase #6 $124.69   4.808%   $130.6851   $130.69   $130.70   $130.70  
  Purchase #7 $104.98   6.806%   $112.1249   $112.12   $112.10   $112.10  
  7 purchase totals = $815.68       $881.0530   $881.05   $881.05   $881.00