It depends. On blotch prone woods such as pine, poplar, cherry, and maple, yes. On oak and walnut, it depends. A pre-stain conditioner can limit absorption and therefore the shade of the stain - it may not be as dark. Pre-stain conditioning will even out the color tone across the piece, and this may or may not be desirable, i.e. for a more rustic look don't condition. End grain, especially for red oak, will absorb much more stain. It can be conditioned by itself, but that can be difficult. I prefer to sand the end grain to ~600 and then burnish it with a polished piece of metal. This closes off the pores and limits absorption.
There are multiple approaches to pre-stain conditioning. The solvent based products I am aware of are simply solvents that are absorbed by the wood grain until saturated, and then begin evaporating. You need to stain within a time window - too early and not much stain is absorbed, too late and more blotching occurs.
Thinned topcoat finishes, such as shellac and lacquer can be used. The problem is they dry quickly, and don't allow the uneven absorption by the wood grain to occur.
The best conditioners are water based, and can be used with solvent or waterbased stains and topcoats. About any water based topcoat can be thinned 50/50 with water and used as a pre-stain conditioner. Also, PVA glues can be used, thinned with water 4-10 parts water to 1 part glue. I use Elmers Glue All 4 parts water to 1 part glue. It is the cheapest and dries clear.
Water based are best because of the long open time and relatively quick dry time. The long open time allows the conditioner to be absorbed unevenly by the grain. the water evaporates leaving more sealer in the more porous areas resulting in more even stain absorption. It will not affect the amount of pigment deposited on the surface for a pigment type stain. It effects the dye portion of the colorant. Solvent poly could also be used (long open time) but has a very long dry time. I have not tested it and don't know the thinning ratio.
Apply the water based conditioner by flooding the surface (spray, brush, sponge, doesn't matter) then using a brush keep spreading and adding until the surface stops absorbing, then wipe it off with dry rags/paper towels and let dry. Sand lightly with either the last grit used or the next one up - remove the raised grain but don't cut too much away. Pine will swell significantly, and leveling the surface will remove the conditioner in spots. It needs a 2nd application. Stain and topcoat with solvent or waterbased products as usual.
Yes, a coat of solvent poly (or a water based finish) can be applied over any of the conditioners described. Allow the solvent conditioner to evaporate completely, and let the water based conditioner dry, then apply the poly.
It is highly recommended to test any new finish or preparation method on scrap first.