If your battery is just low and will take a charge then starting your car up and driving it when the battery was low is okay and safe.
That is as long as your cars battery is taking a charge and will charge back up from the alternator when you're driving the car.
However if the battery is not taking a charge and has to always be jumped to start the car then you will risk being stranded should the battery decide to not take a charge.
If your battery is going bad or is older than 5 years old you really should replace the battery because it can leave you stranded and it makes it harder on the starter when trying to start the engine using a low battery.
I always replace my vehicles battery after every 5 years regardless if it's still holding a charge because it's much better to spend the money and get a new battery than waiting for the battery to completely fail on you and then leaving you stranded.
You might get somewhere where there's no one available to help you jump start your vehicle.
If your battery is constantly going low then your battery terminals could be dirty not allowing the alternator to send enough current to the battery to keep it charged or the battery is getting a dead cell and needs replaced.
Batteries in vehicles should always be replaced every 5 years to prevent a break down.