I get asked this question all the time and it is a hard one to answer. Requiring people to enter information into any field can certainly help ensure they fill it out. But you can also end up with the “keyboard slap” – that is where the user just hits any key on the keyboard to put data in so they can get past this and on to what they need to do. So, if you do need things mandatory, please be very discretionary about which fields and limit that list as much as possible.