No you don't need one. All that will happen it the bottom of your sig, as Ginger says, will get a bit wet/muddy/ grassy. Wipe as much off as you can when you strike camp and when you get home you will need to take your tent out of the bag and give it an air to make sure it is bone dry before you pack it away for the winter.
I would of done that but was told a tale about how cats climbed all over somebody's new tent and literally pierced it all over. I would cry a lot if that happened to mine after spending most of the summer with bin bags over the air beds and sleeping bags I absolutely love this tent
Buy a tarpaulin and put that down under it. As others have said it helps keep the sig clean. It will also help protect it from anything on the ground tearing your sig and it will add an extra layer of insulation between you and the ground.
The cat thing happened to my friend. Her boys were camping in the back garden in the summer holidays and their new kitten climbed over and left a trail of claw-holes. luckily it went under the flysheet first and only went over the inner dome so the tent is still waterproof.