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 $20.51   3.770%   $21.2832   $21.28   $21.30   $21.30  
  Purchase #2 $47.91   4.271%   $49.9562   $49.96   $49.95   $50.00  
  Purchase #3 $144.62   10.449%   $159.7313   $159.73   $159.75   $159.70  
  Purchase #4 $222.46   5.177%   $233.9768   $233.98   $234.00   $234.00  
  Purchase #5 $84.76   3.595%   $87.8071   $87.81   $87.80   $87.80  
  Purchase #6 $27.49   6.566%   $29.295   $29.30   $29.30   $29.30  
  Purchase #7 $53.80   7.804%   $57.9986   $58.00   $58.00   $58.00  
  7 purchase totals = $601.55       $640.0482   $640.06   $640.10   $640.10