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 $117.63   3.570%   $121.8294   $121.83   $121.85   $121.80  
  Purchase #2 $122.71   11.724%   $137.0965   $137.10   $137.10   $137.10  
  Purchase #3 $124.75   11.862%   $139.5478   $139.55   $139.55   $139.50  
  Purchase #4 $172.70   6.152%   $183.3245   $183.32   $183.30   $183.30  
  Purchase #5 $158.48   10.730%   $175.4849   $175.48   $175.50   $175.50  
  Purchase #6 $47.54   12.219%   $53.3489   $53.35   $53.35   $53.30  
  Purchase #7 $26.74   10.269%   $29.4859   $29.49   $29.50   $29.50  
  7 purchase totals = $770.55       $840.1179   $840.12   $840.15   $840.00