I'm no expert on Hardi- I just had it installed on our build a few months ago. Per our contractor, the biggest variable in how it will lay is the underlying framing and sheathing. Are you going over XPS foam board? If so, I'd worry they are sinking the nails too deep and creating waves in your hardi. This may be the problem even if going over wood sheathing. If they can't put if up right, stop them now. If I recall correctly, a 7" reveal is the maximum width before they recommend face nailing in addition to the nails at the top of each plank. I'd hate to have them face nail the whole job to make it look right just because they couldn't put it up properly.
It won't straighten itself out later. I spoke to a few contractors for the hardiplank, and it was obvious which had done it a lot and which had not.

do you think i'd be better off going with the next size smaller reveal? we're not going over foam board, just the wood & hardiwrapping.

I put this on the house I'm building, installing it with my carpenter. We used the 12" Hardiplank, and I was very pleased with everything about it.
The product is not exactly rigid like wood, but I'd say it should be easy enough to lay flat, and keep the joints flush.
After caulking our joints, you can hardly see them.
I'm wondering like the other poster, what is the wall sheathing like under the Hardiplank? We used Huber Zipwall, which is great.
I assume they "blind nailed" the siding when intalling, so the nails don't show. With the 12 inch, we had to nail it top and bottom, then caulk over the bottom nail holes.

Also, and this is important, if a piece of the siding ends near a stud, too near to nail for fear of the siding splitting, you need to screw it down. Do this by first countersinking a hole in the siding with a drill so that when you install the screw you can get the screw head down below the top of the siding. Then dab some caulk on top. Sounds like alot of work, and it is, but that's the price of doing it right.

Get a Hardie rep to look at it ASAP.