Thanks for the comments everyone. Glad you enjoy them..
I was alternating between two Cokin ND Grads- a 0.6 and 0.9 (both soft edge). Everything happens pretty fast when you're out there shooting so unfortunately I can't recall which shot used which ND Grad. Every shot in this post did use one, though.
I did buy a P series filter holder but like many others, I've learned its much easier and faster to handhold them in front of the lens. Shots like #3 & 4 were probably done using the 0.9 ND grad due to the somewhat darker foreground and the brighter skies whereas photo #5 was probably the 0.6 ND grad since I didn't want as much detail in the foreground.
Most of the time, I am resting the fllter against the front of the lens (a circ polarizer BTW) but at least on one occasion, it added some vibration into the photo which I did not notice while reviewing the photo in the field. On the other hand, holding it in front of the lens without touching the lens can introduce reflections. Kind of a balancing act..
The Cokin filters have been great to learn with (I bought them used) but I'm ready to upgrade them to Lee (and perhaps Sing-Ray). I've messed up more than a few shots because portions of my hand showed up in the frame, especially while using the 12-24mm lens. The Lee filters are significantly longer so they should be easier to hold while keeping my hand out of the frame.
In case people haven't seen it, this might be useful:
The Art of Filtration in the Digital Age - Part 2