There are a few schools of thought on this one. Some people say that you don't need a handle, because it's not like you're going to be bouncing super high. Others say that a handle is helpful in keeping your balance and stability while you're rebounder. And still others believe that it's personal preference - if you feel more comfortable with a handle, then go for it!
The main thing to consider is what kind of rebounder you have. If you have a mini-rebounder, then it's probably not necessary to have a handle. But if you have a full-size rebounder, then having a handle can be helpful, especially if you're doing any sort of jumping or other movement on the device.
At the end of the day, it really comes down to your own comfort level and preferences. If you feel like you need a handle to keep your balance or stay stable, then go ahead and use one!