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 $226.73   8.679%   $246.4079   $246.41   $246.40   $246.40  
  Purchase #2 $169.25   13.411%   $191.9481   $191.95   $191.95   $191.90  
  Purchase #3 $158.99   10.641%   $175.9081   $175.91   $175.90   $175.90  
  Purchase #4 $125.46   7.493%   $134.8607   $134.86   $134.85   $134.90  
  Purchase #5 $99.62   4.253%   $103.8568   $103.86   $103.85   $103.90  
  Purchase #6 $147.73   8.203%   $159.8483   $159.85   $159.85   $159.80  
  Purchase #7 $211.81   12.177%   $237.6021   $237.60   $237.60   $237.60  
  7 purchase totals = $1,139.59       $1,250.4320   $1,250.44   $1,250.40   $1,250.40