I love getting tipped big, but one of the things I will never do to get tipped is beg. Begging for tokens is one of the things that turns guys off the quickest. In my experience as a cam model, if your viewers see you as some kind of charity case, they are less likely to stick around. You'll soon find that fewer and fewer of your regular visitors return to hangout with you again, and you'll be ending up in a quiet, dead chat room.
Begging is a total desperation move, and guys see it for exactly that. Webcam models who do it are quickly categorized as pity cases, and there's no bigger boner killer than a pity case. Cam site members want to see you as a confident, sexually bold hottie who's camming because she just wants to have fun, not because she needs money. Anything you do to contradict that image will end up hurting you in the long run.
Having said all of that, it's not even like begging is totally ineffective. I've heard of girls who were able to guilt their chat rooms to tip them due to needing rent money, or money for food, but it was always a short-term gain, long-term loss type of deal. Sure, they might be tip you out of pity, but that pity will most likely sap any sexual arousal for you that may have once been there. My advice is, definitely don't beg for tips.