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 $9.38   6.361%   $9.9767   $9.98   $10.00   $10.00  
  Purchase #2 $74.50   6.543%   $79.3745   $79.37   $79.35   $79.40  
  Purchase #3 $146.30   12.616%   $164.7572   $164.76   $164.75   $164.80  
  Purchase #4 $107.75   8.241%   $116.6297   $116.63   $116.65   $116.60  
  Purchase #5 $246.42   3.750%   $255.6608   $255.66   $255.65   $255.70  
  Purchase #6 $48.32   10.368%   $53.3298   $53.33   $53.35   $53.30  
  Purchase #7 $248.37   4.162%   $258.7072   $258.71   $258.70   $258.70  
  7 purchase totals = $881.04       $938.4359   $938.44   $938.45   $938.50