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 $17.70   13.235%   $20.0426   $20.04   $20.05   $20.00  
  Purchase #2 $149.71   12.510%   $168.4387   $168.44   $168.45   $168.40  
  Purchase #3 $228.64   3.189%   $235.9313   $235.93   $235.95   $235.90  
  Purchase #4 $15.66   4.139%   $16.3082   $16.31   $16.30   $16.30  
  Purchase #5 $225.39   8.845%   $245.3257   $245.33   $245.35   $245.30  
  Purchase #6 $89.75   11.724%   $100.2723   $100.27   $100.25   $100.30  
  Purchase #7 $192.76   11.576%   $215.0739   $215.07   $215.05   $215.10  
  7 purchase totals = $919.61       $1,001.3927   $1,001.39   $1,001.40   $1,001.30