While I agree with the other posts as to the best way to winterize, you really asked if it was possible to get ALL the gas out. There are times when that is neccessary as when storing the bike indoors (some might say that techically its OK with a full tank, - yikes, no way I'm storing any amount of gas in my house. That's my policy and I don't give a dang). I just emptied the tank in order to flush it and inspect the petcock screens. I have my own reasons for this - irrelevant to the procedure.
I got some fuel line long enough to run from the petcock into a safe container. Drained the tank with the valve set to reserve. Then placed a tray under the tank (tank is off the bike) and removed the petcock. More fuel splashed out the opening in the tank. Shaked the tank, even more came out. This can be very messy and very dangerous - there is both both liquid and vapor gasoline. Even after this I'm not sure that ALL the gas was out. As mentioned, any time the tank is dry, rust is a possiblity. The process above can be very dangerous, I don't recommend it, its not neccessary, don't do it. It's just what I did, again, for my own reasons.
By the way, both the main and reserve tubes have fine screens in them. There is another screen in the fuel line at the carb. It would take a lot of debris to clog them and I think the carbs are pretty well protected from debris.