I'd been using the K5 when I picked up a K-1. I found the camera alone is taller in the body so it could be that you may be okay without a battery grip.
However as I'd been using battery grips for my previous three bodies I decided I'd end up missing it if I didn't get one so I decided to grab one for the K-1. I use the camera vertically probably a third of the time so it can be handy for this. The button placement doesn't completely mirror the camera body so if back-button focus is something you do it is something you should be aware of. It's not insurmountable just a bit annoying.
I like the idea of the extra battery capacity that the grip provides and the ability to carry a third SD card, though the dual SD slots of the K-1 has reduced the importance of this capability.
I use my K-1 with a hand strap from Joby and find the battery grip provides a more flexible hand hold using this combo. I also carry the camera by the battery grip sometimes to change up the hand position which is handy when I'm using one of my heavier lenses. A problem with this combo is from time to time the bottom of the hand strap sits across the battery grip shutter button. I'm aware of the problem and have adapted some checks for this though from time to time I get an exposure I didn't know I had. Usually of my foot or something like that, so I may as well blame the gear.
If you find the battery grip gives you more than just extra real estate for the hand you might be best off getting one for the K-1 especially if you do long trips as it will save the amount of battery changes needed. If you only need the battery grip to provide extra space for gripping the camera, I think you'll find the K-1 body a better fit without needing the grip, unless you've got very large hands.
Good luck with your decision, rest assured you're going to enjoy the K-1.
Tas