Is it bad for the battery to keep a cell phone on charge most of the time?

yip, it frys the battery and you will find that when you go to use it again the battery life will be really short. Same thing with Laptops that are kept on charge all the time without taking the battery out first.

You should always try to totally discharge the battery before you charge the device up again (whatever the product may be).

Yes technically it is. All batteries work pretty much the same way, it is best to use them until drained, then recharge them completely. However in the real world this is not really a good option so you can just continually charge them and use the cell phone as needed. The only draw back to this is if you need to take the cell with you without a charger and use it a lot it will not hold the charge very long.

Yes it is as overcharging reduces the battery life. Let the battery meter go down to less than half before recharging. Do not recharge for more than two hours at a time.

You can 'over charge' a battery, making it useless. Take it off the charger from time to time.