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 $15.55   5.592%   $16.4196   $16.42   $16.40   $16.40  
  Purchase #2 $90.54   13.639%   $102.8888   $102.89   $102.90   $102.90  
  Purchase #3 $221.35   4.498%   $231.3063   $231.31   $231.30   $231.30  
  Purchase #4 $217.34   6.627%   $231.7431   $231.74   $231.75   $231.70  
  Purchase #5 $50.48   8.291%   $54.6653   $54.67   $54.65   $54.70  
  Purchase #6 $127.47   4.382%   $133.0557   $133.06   $133.05   $133.10  
  Purchase #7 $63.54   10.605%   $70.2784   $70.28   $70.30   $70.30  
  7 purchase totals = $786.27       $840.3572   $840.37   $840.35   $840.40