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 $83.41   6.109%   $88.5055   $88.51   $88.50   $88.50  
  Purchase #2 $144.59   10.523%   $159.8052   $159.81   $159.80   $159.80  
  Purchase #3 $141.86   6.229%   $150.6965   $150.70   $150.70   $150.70  
  Purchase #4 $98.61   8.711%   $107.1999   $107.20   $107.20   $107.20  
  Purchase #5 $13.51   5.264%   $14.2212   $14.22   $14.20   $14.20  
  Purchase #6 $65.91   11.712%   $73.6294   $73.63   $73.65   $73.60  
  Purchase #7 $229.74   4.240%   $239.481   $239.48   $239.50   $239.50  
  7 purchase totals = $777.63       $833.5387   $833.55   $833.55   $833.50