I was thinking about putting a J Pole in the attic, just for times I need to lower the mast, which should only be for a couple days. I've read that J Poles should be grounded and I wanted some opinions on that.
A j-pole doesn't need a ground plane like a quarter wave does. But the coax shield from the antenna to the radio is a chassis ground. It's part of what makes the j-pole work so well.
Since you plan to place this in the attic, a ground for the mast as lightning protection probably is not necessary, and you probably won't need a lightning arrestor installed inline on the coax either. In fact, for an attic install, you really don't need a mast, at least in the traditional sense.
I have two j-poles from Ed Fong. They're a wire ladder designed to be installed inside thin wall PVC piping, and there is little chance of them being a lightning rod if I were to mount them on top of the house. I'd still place a lightning arrestor inline for outside use like that.
My amateur radio club uses a j-pole fabbed out of 600 ohm tv antenna lead. It's not in any kind of encasement. They just fasten it to a 15' pole with some rubber bands and tie the pole to one of the legs of an outdoor shelter. The only ground there is the coax from the antenna to the radio.
I would check continuity on the coax shield from the antenna to the radio just to be safe.