Online banking makes tracking expenses much easier than the days of checkbook registries, but it doesn't ensure you're safe from overdrawing your account. Spending money we don't have always comes with a price, whether in the form of credit card interest of overdraft protection fees from your bank.
If an outstanding check slips your mind and it bounces because you spent money that you didn't really have, overdraft protection can cost about $20-$30 each time you use it. Instead of opting for this type of protection, ask your bank to link your checking account to your savings account in order to save on fees. The cost of transferring funds from one account to the other is typically a fraction of a standard overdraft fee.