You don't have to destroy, burn, and give your hair split ends. If straitening your hair is brutal, then stop it. Just wash it and make sure it's clean and shiny. And brush it (or comb it). I don't like girls who don't brush or comb their hair and leave it in tangles. Curls are fine, but tangles are a turn off. A bed head that is moppy is totally ugly.
If it is clear that you take care of your hair, then that's good enough. But please try to make your hair look feminine, and take pride in your hair; that is such a turn on. It doesn't matter if it is curly: as long as it's shiny and untangled.
It depends not only on the guy, like Avalace said, but also your looks. Hair color and facial features have a lot to do with how your hair looks. Personally, I've always felt that blond hair was terrible for curling, brunette was ok, and red was awesome. But that can change depending on the length and facial features.
Ideally, I prefer a girl who can pull off a short hairstyle. I've always felt that any girl who is able to look beautiful with a short hairstyle is truly a natural beauty.
For me, and remember that each guy is different, I like longer hair. Like say shoulder length or longer. I could care less if it is straight or curly. Blonde, red, or brown. To me that makes no difference.
In my experience, each girl knows how to do her hair in a way that she like and feels good with. And that is what is an even bigger turn on for me. A girl that is confidant in the way she looks, not just her hair, but in general.
The girl I dated longer than any in my life, only three years or so, had the curliest, but frizzy sometimes, red locks you have ever seen. She used to have real trouble with it and wanted to straighten it all the time, but then it looked poofy. I always liked it other than the fact that it was hard to run my fingers through, which I love to to when I am just lying with a girl. Personally, though it used to bring her to tears sometimes, it never bothered me when she left it all curly.
Like other said different guy like different style, I personally ok with straight or curly but prefer long hair, but the girl herself have to have the right jewelries, taste in clothes, right make up plus the hairstyle to be able to stand out from a crowd, doesn't really feel attractive if she had the best hair in the world but with bad taste in clothes, dry lips, sleepy black eyes, dry skin etc.