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 $249.35   3.561%   $258.2294   $258.23   $258.25   $258.20  
  Purchase #2 $52.90   13.851%   $60.2272   $60.23   $60.25   $60.20  
  Purchase #3 $188.46   7.615%   $202.8112   $202.81   $202.80   $202.80  
  Purchase #4 $92.35   4.429%   $96.4402   $96.44   $96.45   $96.40  
  Purchase #5 $65.54   3.491%   $67.828   $67.83   $67.85   $67.80  
  Purchase #6 $5.95   11.308%   $6.6228   $6.62   $6.60   $6.60  
  Purchase #7 $191.86   6.675%   $204.6667   $204.67   $204.65   $204.70  
  7 purchase totals = $846.41       $896.8255   $896.83   $896.85   $896.70