we were told 4H for constructions, and 2H for filling lines in. i wouldn't recommend B pencils, as they appear grainy and smudge a lot. i'm not sure it matters that much, once it is clear and (as much as possible) correct.
I use a 3H for light lines and a H for normal ones, if you want to go over something heavily I suggest an F, it's halfway between a HB and a H, and it's buildable - not to dark or light. Don't use B pencils for TG, they will smudge and ruin the whole thing!