You're installing a giant toaster on your roof. Definitely use a GFCI.

According to the pictures, the roof-deicing units sold up here are installed in a zig-zag configuration along the bottom edge of the shingled roof. Makes sense to me, since the goal is to create drainage channels.

[Aside: I think I'll install a GFCI for the block heater on my wife's car. We have a single carport attached to the old house with no fire barrier. And the news just had a story about a block-heater-cord fire that trashed a house. Yikes. Could be $15 well spent.]