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 $13.26   13.212%   $15.0119   $15.01   $15.00   $15.00  
  Purchase #2 $224.63   10.742%   $248.7598   $248.76   $248.75   $248.80  
  Purchase #3 $153.46   9.611%   $168.209   $168.21   $168.20   $168.20  
  Purchase #4 $232.91   4.620%   $243.6704   $243.67   $243.65   $243.70  
  Purchase #5 $85.70   13.630%   $97.3809   $97.38   $97.40   $97.40  
  Purchase #6 $62.76   10.243%   $69.1885   $69.19   $69.20   $69.20  
  Purchase #7 $156.48   10.206%   $172.4503   $172.45   $172.45   $172.50  
  7 purchase totals = $929.20       $1,014.6708   $1,014.67   $1,014.65   $1,014.80