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 $193.36   12.825%   $218.1584   $218.16   $218.15   $218.20  
  Purchase #2 $117.95   13.575%   $133.9617   $133.96   $133.95   $134.00  
  Purchase #3 $86.29   3.108%   $88.9719   $88.97   $88.95   $89.00  
  Purchase #4 $125.89   11.531%   $140.4064   $140.41   $140.40   $140.40  
  Purchase #5 $47.95   12.357%   $53.8752   $53.88   $53.90   $53.90  
  Purchase #6 $219.54   7.252%   $235.461   $235.46   $235.45   $235.50  
  Purchase #7 $107.70   6.304%   $114.4894   $114.49   $114.50   $114.50  
  7 purchase totals = $898.68       $985.3240   $985.33   $985.30   $985.50