Does not hurt if you wish to but means having to wait up to five minutes in morning for modem to boot up and internet to be available. Myself I mostly leave the modem on 24/7 except when there is a lightning storm happening close by as the lightning can destroy the modem. During lightning storms I not only remove the power cable from the modem but also the phone line cable as well as lightning spikes can come in through either or both of those cables. I also remove power and antenna cable as well from the TV if I am lucky enough to be home at the time.
A good reason to leave modem on is if your mobile phone is configured to switch to WiFi when home that will save phone data as the phone will get data via modem for free rather than through phone data.
