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 $8.89   11.226%   $9.888   $9.89   $9.90   $9.90  
  Purchase #2 $157.39   13.141%   $178.0726   $178.07   $178.05   $178.10  
  Purchase #3 $45.67   3.485%   $47.2616   $47.26   $47.25   $47.30  
  Purchase #4 $132.30   10.428%   $146.0962   $146.10   $146.10   $146.10  
  Purchase #5 $30.49   6.420%   $32.4475   $32.45   $32.45   $32.40  
  Purchase #6 $130.43   6.806%   $139.3071   $139.31   $139.30   $139.30  
  Purchase #7 $103.40   4.677%   $108.236   $108.24   $108.25   $108.20  
  7 purchase totals = $608.57       $661.3090   $661.32   $661.30   $661.30