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 $58.61   10.248%   $64.6164   $64.62   $64.60   $64.60  
  Purchase #2 $13.57   3.210%   $14.0056   $14.01   $14.00   $14.00  
  Purchase #3 $28.30   3.391%   $29.2597   $29.26   $29.25   $29.30  
  Purchase #4 $56.86   4.403%   $59.3635   $59.36   $59.35   $59.40  
  Purchase #5 $51.75   10.145%   $57   $57.00   $57.00   $57.00  
  Purchase #6 $237.62   4.269%   $247.764   $247.76   $247.75   $247.80  
  Purchase #7 $60.87   10.746%   $67.4111   $67.41   $67.40   $67.40  
  7 purchase totals = $507.58       $539.4203   $539.42   $539.35   $539.50