ARe you talking about 2 strand twists or the kind that run along your scalp like cornrows?
If you want big fat juicy twists you don't necessarily need to part perfectly, but you do need to grab more hair and twist looser. You also need to decide at what stage your hair expands most. What I mean is that when I was natural, had a texturizer and even now, my hair goes flat when it's wet. A friend of mine was the opposite. Water made her hair pouf up even more. My hair poufs and get's fat when it's dry so I'd do the twists wet and then take them loose when they dried.
The same is true if you're talking about twist that run along the scalp. For me, parting is essential because it helps with definition. Use more hair and bigger parts.
HTH