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 $231.78   11.431%   $258.2748   $258.27   $258.25   $258.30  
  Purchase #2 $111.64   13.501%   $126.7125   $126.71   $126.70   $126.70  
  Purchase #3 $236.26   5.168%   $248.4699   $248.47   $248.45   $248.50  
  Purchase #4 $25.87   12.752%   $29.1689   $29.17   $29.15   $29.20  
  Purchase #5 $200.25   3.386%   $207.0305   $207.03   $207.05   $207.00  
  Purchase #6 $72.65   13.522%   $82.4737   $82.47   $82.45   $82.50  
  Purchase #7 $46.50   6.482%   $49.5141   $49.51   $49.50   $49.50  
  7 purchase totals = $924.95       $1,001.6444   $1,001.63   $1,001.55   $1,001.70