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 $248.79   8.141%   $269.044   $269.04   $269.05   $269.00  
  Purchase #2 $59.73   5.131%   $62.7947   $62.79   $62.80   $62.80  
  Purchase #3 $42.68   6.445%   $45.4307   $45.43   $45.45   $45.40  
  Purchase #4 $18.26   7.204%   $19.5755   $19.58   $19.60   $19.60  
  Purchase #5 $236.57   9.238%   $258.4243   $258.42   $258.40   $258.40  
  Purchase #6 $209.39   8.302%   $226.7736   $226.77   $226.75   $226.80  
  Purchase #7 $94.65   13.692%   $107.6095   $107.61   $107.60   $107.60  
  7 purchase totals = $910.07       $989.6523   $989.64   $989.65   $989.60