First and foremost, it's a matter of opinion, experience, budget, and to some degree, taste.  All conditioned by the fact that if you're crashing all the time and you have some nice 4 pound 3/32" wood -- save it for when you don't crash any more, and build a fuselage out of it!

There's a difference between straight grain and grain type.  Straight grain is (mostly) assumed -- certainly I'd want to cut any sticks from clear (not knots or wormholes) straight-grained balsa.  For ribs or sheeting something I'd be willing to put up with a bit of waviness, maybe.

The type of grain determines the strength properties of the wood across the grain.  A grain bends across the grain easily, which makes it the stuff to use for sheeting curved surfaces, and absolutely the wrong stuff to use for all-sheet stabilizers and control surfaces.  C grain is the opposite -- it's quite stiff across the grain, but breaks if you try to bend it.  So use it for flaps, elevators, and stabs.  B grain is (literally) in the middle; it's good where things don't matter as much; i.e., ribs, straight fuselage sides, formers, whatnot.
This page classifies grain nicely.

For weight, first, keep in mind that for anything sheeted you can go down a size and up a bit in density and come out OK, or you can go up in density and fill the part with lightening holes.  If you're building to magazine plans and the wood density isn't called out, I'd try to go with contest (6lb or less) for any large sheeted areas.  Lighter for sheeting, on the heavier end of the spectrum for ribs and formers.  For spars I'd want to use at least 8 lb material, and I wouldn't sniff at using 12 pound if I had it.