Yes plywood warps in time with exposure to humidity. If your shelf gets soaking wet, even hardwood plywood will warp badly & peel apart. Painting or staining is your personal choice for the look you want. Poplar will be very smooth and capable of glossy finish so you should use a high quality synthetic brush or it will show lines. Use a natural bristle, quality brush if you stain. Poplar is a "soft" wood, porous so if you stain, first apply a coat of conditioner to keep it from looking mottled. If you paint, apply a primer (tinting it will help speed things) and apply as many coats as gets the look you want. To retard moisture, you can apply an oil-base top coat. If this shelf will get alot of use and you want less maintenance, apply two coats of polyurethane.