I was planning just to polyurethane it without staining. Do I still need to use a pre-stain conditioner?
No. The point of a pre-stain conditioner is to prevent stained wood from looking mottled or blotchy because some areas are more porous and absorb more stain than other areas. The conditioner reduces that effect by sealing the wood to some degree so that it absorbs stain more evenly.
Since you're not going to stain your project, there's no need to apply a pre-stain conditioner.
Perhaps you're worried about your polyurethane finish being absorbed into the wood unevenly? That might happen, but since the finish is clear I don't think you'll see an effect on the color. Don't take my word for it, though... apply some polyurethane to some scrap pieces of the wood you're using and see how it looks. In fact, unless you've used the same finish on the same wood in the past, it's always a good idea to try out the finish on some scraps before you do your whole project so that you know exactly how the finish will look. It's easier to switch to a different product before you've started finishing your project than in the middle of the process.
Also, will the poly eliminate the insect-repelling effects of cedar?
I'd expect it to significantly reduce any cedar scent coming from the wood. Polyurethane forms a film on the surface of the wood, and that film will most likely trap the oils that give cedar it's smell inside. If you want to finish the wood but still want some of that scent, then look for ways to incorporate some unfinished wood into the project. It's not a good idea to finish one side of a piece of wood without also doing the other, since the unfinished side will be more exposed to seasonal changes in humidity, and that can make the wood warp. But you could still line the inside of your cabinets with some unfinished cedar, for example, and you could use unfinished boards for the slats that the box spring and mattress will sit on.
