I’m just curious if it’s necessary/more secure to use the onscreen keyboard for keying in passwords when using wifi? Does it help? Or is it just good to have a robust password?
Depends on what device you’re trying to connect to wifi. For an ordinary laptop computer, I’d say that there are minor benefits and drawbacks to it.
BENEFIT: you bypass most kinds of hostile keystroke loggers that might already be installed on your computer, trying to capture your password
DRAWBACK: If somebody can see your screen, they might be able to follow the mouse pointer to figure out your password that way.
Logically, the best of both worlds would be to alternate - one character on the normal keyboard, one on the on-screen and so on. 
That way, they at least need to have both visual observation and the keystroke logger, and even so might have difficulties reconstructing the proper order.
Confusing… if you’re asking about protecting yourself from keyloggers and the like, chrisk’s suggestion is the way to go. However, this is for a locally compromised computer.
Your post suggests that you want to increase security WHILE connecting to the Wifi network, and possibly encrypting the password as it flies through the air to the access point. I can tell you that neither typing on a keyboard nor using an on-screen keyboard will offer you any more (or less) benefit.
As long as you are connecting to a WPA (or preferably, WPA2) enabled network, you should be fine.
Even the WEP encryption technology, even though it has been cracked, will deter most, if not all, casual wireless bandits. Simply put, you’d need some fairly specialized software to sneak a peek into the encrypted password flying through the air 