Depends on how you are finishing the alder body. If you want to shoot a gloss nitro top finish, you can do that on top of 2-4 coats of sanding sealer; with alder the coats of sanding sealer can be used in lieu of grain filler as there's hardly anything to fill.
If you were going to use tung oil or something similar, wouldn't need the sanding sealer.
Maple necks can just be shot directly with nitro; I like to use some Fender Neck Amber to "vintage" them up first; no need for the sealer at all on the maple neck.
How exactly were you planning on finishing it, in terms of color/stain/dye and top finish?