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 $37.47   7.517%   $40.2866   $40.29   $40.30   $40.30  
  Purchase #2 $21.80   6.670%   $23.2541   $23.25   $23.25   $23.30  
  Purchase #3 $28.50   9.531%   $31.2163   $31.22   $31.20   $31.20  
  Purchase #4 $42.95   8.785%   $46.7232   $46.72   $46.70   $46.70  
  Purchase #5 $62.35   11.491%   $69.5146   $69.51   $69.50   $69.50  
  Purchase #6 $81.70   3.857%   $84.8512   $84.85   $84.85   $84.90  
  Purchase #7 $235.68   3.699%   $244.3978   $244.40   $244.40   $244.40  
  7 purchase totals = $510.45       $540.2438   $540.24   $540.20   $540.30