I rarely use anything on oak, sometimes on maple, but on any of those woods that Bruce mentioned, yes, all the time. Those are the ones that give you blotchy stain and that's all you want conditioner for. Even though I still prefer a "uniforming coat" or "uniforming finish" application. That's big words for a "spit coat" and that's still best and easiest for me.
According to The Master, Bob Flexner, in his book Understanding Wood Finishing, a sanding sealer is not a good practice for most finishing. They just add zinc stearate to their regular finish to make Sanding Sealer. Zinc stearate is a mineral soap and slippery. It makes for great sanding but poly's don't stick well to anything slippery. Check out pages 133-4 of his book.
For a real wild sounding method if you get stuck not testing and finding out that you can't get the stain color dark enough, you can put on a coat of poly--I know that this worked with oil-based but never tried it with water based--and put another coat of stain on scuff sanded poly. It's been that way on a floor for more than 5 years with 1 coat stain, 1 poly, 1 stain, 1 poly, 1 stain and finally poly. I was doing some playing with effects and it came out great and lasted so far, again in a high traffic area floor. Please, don't try this at home. LOL
And I know you are going to ask about filling the grain on oak. Why not use a grain filler. Behlen makes a great one, among others. It's made for filling pores, not gaps, tear-out, etc. Not that I would know anything about those last items. LOL