Do you use a tip? If so, what number tip?
Yes... petal tip of your choice.
I'd use a flat tip like you'd use to do a basketweave. Start at the top and do one row, then work your way down the cake. You'll need a steady hand and a good turntable.
Yeah the flat tip - I think the carnation tip would work best and I *think* the # is 150. Unless you want to buy another tip - then get one that is about 1" wide. Seems to me the carnation tip is about 1/2".
CWR41 mentions a petal tip - I think she is referring to a rose tip and that will give you a fat end w/a narrow end - so your row upon row will not look as good as using the carnation tip.
AA tip is make sure you don't over fill your bag, or you will loose control of the bag. Which is absolutely essential for this design. I have abnormally tiny hands. It takes me forever to do something like this...I'm looking for a picture but can't seem to find it.
Steady hand is key