OK, I admit it. Even now I will do it if there is no multi-meter around.
I do it all the time for the boy's toys and battery operated stuff. Done it since I was a little boy. My 12 year old does it too! I agree with Amir, you can get pretty darn accurate with voltage +/-0.2V with practice- ouch!
Last time I changed all Dad's smoke detector 9V backup batteries, I tested them all with my tongue and ranked them in order. He of course got out the multimeter to confirm...