Overnight companions' tips are already included in your package price.  
Porters will expect a tip, and staffs are rewarded for good service.  Use your discretion if you have received particularly good service then leave a tip if you wish.
Thanks, and a smile, are also much appreciated. 

There are no hard and fast rules, and the standard British 10 percent let alone the American 15 percent would be far too much on a large bill. 
Between 10 and 50 babt is adequate in almost every circumstance. Expensive hotels will automatically include a service charge on the bill.