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 $38.99   8.838%   $42.4359   $42.44   $42.45   $42.40  
  Purchase #2 $75.82   4.712%   $79.3926   $79.39   $79.40   $79.40  
  Purchase #3 $159.68   12.482%   $179.6113   $179.61   $179.60   $179.60  
  Purchase #4 $231.52   3.595%   $239.8431   $239.84   $239.85   $239.80  
  Purchase #5 $94.36   12.874%   $106.5079   $106.51   $106.50   $106.50  
  Purchase #6 $133.29   6.583%   $142.0645   $142.06   $142.05   $142.10  
  Purchase #7 $176.83   9.584%   $193.7774   $193.78   $193.80   $193.80  
  7 purchase totals = $910.49       $983.6327   $983.63   $983.65   $983.60