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 $108.76   13.685%   $123.6438   $123.64   $123.65   $123.60  
  Purchase #2 $115.25   9.216%   $125.8714   $125.87   $125.85   $125.90  
  Purchase #3 $200.88   9.258%   $219.4775   $219.48   $219.50   $219.50  
  Purchase #4 $23.81   6.778%   $25.4238   $25.42   $25.40   $25.40  
  Purchase #5 $11.41   6.580%   $12.1608   $12.16   $12.15   $12.20  
  Purchase #6 $151.67   8.639%   $164.7728   $164.77   $164.75   $164.80  
  Purchase #7 $223.53   3.860%   $232.1583   $232.16   $232.15   $232.20  
  7 purchase totals = $835.31       $903.5084   $903.50   $903.45   $903.60