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 $248.49   8.646%   $269.9744   $269.97   $269.95   $270.00  
  Purchase #2 $225.57   6.205%   $239.5666   $239.57   $239.55   $239.60  
  Purchase #3 $89.63   3.439%   $92.7124   $92.71   $92.70   $92.70  
  Purchase #4 $14.86   12.302%   $16.6881   $16.69   $16.70   $16.70  
  Purchase #5 $205.44   10.482%   $226.9742   $226.97   $226.95   $227.00  
  Purchase #6 $121.51   11.514%   $135.5007   $135.50   $135.50   $135.50  
  Purchase #7 $13.53   6.310%   $14.3837   $14.38   $14.40   $14.40  
  7 purchase totals = $919.03       $995.8001   $995.79   $995.75   $995.90