All toilets should be installed by a professional plumber to avoid poor sanitation. Having poor sanitation from an unproperly installed toilet can cause a life-threatening issue to all the people in the home. If something were to break while being installed you take the risk of having the bathroom flooded with raw sewage.
Having a new toliet installed in your home will also help to add value to your home and, avoid any violations. Newer toliets are designed to conserve energy and save you money, older toliets tend to use more water every flush.