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 $131.78   6.230%   $139.9899   $139.99   $140.00   $140.00  
  Purchase #2 $189.52   3.628%   $196.3958   $196.40   $196.40   $196.40  
  Purchase #3 $43.88   8.665%   $47.6822   $47.68   $47.70   $47.70  
  Purchase #4 $74.54   5.155%   $78.3825   $78.38   $78.40   $78.40  
  Purchase #5 $61.47   6.277%   $65.3285   $65.33   $65.35   $65.30  
  Purchase #6 $42.98   6.647%   $45.8369   $45.84   $45.85   $45.80  
  Purchase #7 $65.83   10.169%   $72.5243   $72.52   $72.50   $72.50  
  7 purchase totals = $610.00       $646.1401   $646.14   $646.20   $646.10