i'm a bit confused on sealers, should they be used only if staining or always before the poly is put down?

It depends upon the the finish you are using. You can usually use oil based polyurethane without a sealer. However, sealers dry faster and usually knock a day off the job. With waterbased finish we always use a sealer first. If staining we do not use a sealer; we will stain then apply the first finish coat whether it be oil or water. However, I have heard of many people sealing after stain.