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 $220.68   8.691%   $239.8593   $239.86   $239.85   $239.90  
  Purchase #2 $109.96   11.757%   $122.888   $122.89   $122.90   $122.90  
  Purchase #3 $141.54   4.150%   $147.4139   $147.41   $147.40   $147.40  
  Purchase #4 $186.89   8.709%   $203.1663   $203.17   $203.15   $203.20  
  Purchase #5 $72.31   12.764%   $81.5396   $81.54   $81.55   $81.50  
  Purchase #6 $241.84   10.432%   $267.0687   $267.07   $267.05   $267.10  
  Purchase #7 $26.91   3.421%   $27.8306   $27.83   $27.85   $27.80  
  7 purchase totals = $1,000.13       $1,089.7664   $1,089.77   $1,089.75   $1,089.80