Your hair ain't going to be curly anymore, when you actually got dreads.
You can't do anything with how your hairs are made biologially, but the hairs are dead when outside of your head. Dreads are straight cause it's hairs that are matted together. Bleaching helps in making hair straight too, which again make it easier to dread. Since unconditioned bleached hair is dry and tangly.
I have the same type of curly 3c hair and pony tails for us just look like big bushy balls of hair...hehe.
If you dread your head, the dreads will be straight as LeRemy pointed out. You may have a curly wispy end though.
I have undercut all four of my sets one way or another. I have dreaded first, then undercut. I have never undercut prior to dreading.
With that, I think undercutting before put in dreads would be the best way to go. 1) You will have a straighter line for your cut. 2) You undercut won't cut into existing dread roots.
My current undercut in just in the back and the line is wavy. In doing the undercut after the dreads, I always nicked a bit of the remaining, "on the line," dreads at the root. Not ideal. Also, in maintaining the undercut, I still nick a "good dread" each time I cut Hehe...it never fails.
So, if you know for sure that you want an undercut before you dread...I would undercut now. That's what I'll do when I ditch this set and grow another