Questions about dyeing cotton....

[smilewithme]

I am going to participate in the flat/handwash challenge this year, and wanted to dye a few. And these questions might depend on the dye, but, since it is my first time, I might just pick up a ready made kit.
So...my questions...
Do I prep the cotton first? I'm using birdseye, flannel, and bamboo terry.
Do you always "heat set" die by placing it in a warm oven? The friend that will be helping me through this says she has never, ever done this. However, I read that it helps set the die. Plus, I doubt anything that she has done was treated as badly as I treat my diapers in the machine you know what I mean. Thanks!