The clips work really well and much faster than pinning. I use them all the time. Having said that, for a tarp ridgeline with slippery materials I still prefer pinning perpendicular to the seam. The clips will still allow some differential slipping between the materials whereas the pins do a better job of keeping them aligned end-to-end. I know that some people use timing marks with success, I just find that pins work better for me in that particular application. By putting the pins inside the seam allowance you won't have any issues, and I think the fabric heals itself from pin pricks pretty well, anyway.

The clips will definitely help you make quick work of hemming the perimeter where two fabrics potentially feeding at different rates isn't an issue.