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 $57.40   12.834%   $64.7667   $64.77   $64.75   $64.80  
  Purchase #2 $229.28   13.851%   $261.0376   $261.04   $261.05   $261.00  
  Purchase #3 $90.67   12.301%   $101.8233   $101.82   $101.80   $101.80  
  Purchase #4 $118.51   6.412%   $126.1089   $126.11   $126.10   $126.10  
  Purchase #5 $214.56   10.274%   $236.6039   $236.60   $236.60   $236.60  
  Purchase #6 $70.72   8.866%   $76.99   $76.99   $77.00   $77.00  
  Purchase #7 $113.80   7.745%   $122.6138   $122.61   $122.60   $122.60  
  7 purchase totals = $894.94       $989.9442   $989.94   $989.90   $989.90