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 $19.57   11.358%   $21.7928   $21.79   $21.80   $21.80  
  Purchase #2 $137.90   8.382%   $149.4588   $149.46   $149.45   $149.50  
  Purchase #3 $100.95   7.247%   $108.2658   $108.27   $108.25   $108.30  
  Purchase #4 $77.25   5.436%   $81.4493   $81.45   $81.45   $81.40  
  Purchase #5 $158.95   10.545%   $175.7113   $175.71   $175.70   $175.70  
  Purchase #6 $96.98   5.655%   $102.4642   $102.46   $102.45   $102.50  
  Purchase #7 $48.65   10.395%   $53.7072   $53.71   $53.70   $53.70  
  7 purchase totals = $640.25       $692.8494   $692.85   $692.80   $692.90