I'd say don't remove the sticker. Or remove it and attach it to something that will act as a nice base, and still allows you to move it around. The idea of permanently affixing a wireless charger somewhere doesn't make sense to me--the ability to move it to wherever you need it at the time would be very important to me.
I think charging efficiency depends on how thick the case is, as well as what material it's made of. I have found that typical TPU cases charge fine, but I haven't compared the charge rate with and without cases.
I have a TechMatte wireless charger it works great with a 2amp plug. I keep it on the nightstand next to my bed for overnight charging. This charger keeps it at 100% without over charging and heating. It might get slightly warm if I have a case on it. But my wife uses it now with her S4 since her charging port broke. I'm getting her a Moto X 2nd gen.... Lucky lol
I have two wireless chargers for my Nexus 5. One I have on the shelf right by my bed at home and the other one I have on my desk at work. Both of them work great for me.