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 $185.63   12.147%   $208.1785   $208.18   $208.20   $208.20  
  Purchase #2 $218.84   11.194%   $243.3369   $243.34   $243.35   $243.30  
  Purchase #3 $43.89   8.487%   $47.6149   $47.61   $47.60   $47.60  
  Purchase #4 $145.31   4.324%   $151.5932   $151.59   $151.60   $151.60  
  Purchase #5 $132.40   3.769%   $137.3902   $137.39   $137.40   $137.40  
  Purchase #6 $32.69   6.861%   $34.9329   $34.93   $34.95   $34.90  
  Purchase #7 $158.41   10.628%   $175.2458   $175.25   $175.25   $175.20  
  7 purchase totals = $917.17       $998.2924   $998.29   $998.35   $998.20