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 $23.62   4.503%   $24.6836   $24.68   $24.70   $24.70  
  Purchase #2 $186.95   3.406%   $193.3175   $193.32   $193.30   $193.30  
  Purchase #3 $151.53   8.190%   $163.9403   $163.94   $163.95   $163.90  
  Purchase #4 $24.36   12.183%   $27.3278   $27.33   $27.35   $27.30  
  Purchase #5 $90.74   11.575%   $101.2432   $101.24   $101.25   $101.20  
  Purchase #6 $60.59   13.182%   $68.577   $68.58   $68.60   $68.60  
  Purchase #7 $19.76   3.676%   $20.4864   $20.49   $20.50   $20.50  
  7 purchase totals = $557.55       $599.5758   $599.58   $599.65   $599.50