Say I have an .fx file with two techniques, TechniqueA and TechniqueB,

TechniqueA uses these constants passed by the engine:
const float4x4 matWorld;
const float4 vecSunDir;
const float4 vecSkill1;
const float4 vecSkill5;

TechniqueB only uses these constants:
const float4 vecSunDir;
const float4 vecSkill1;

If the hardware doesn't support TechniqueA, will the engine still bother setting/passing those unnused constants (and thus slow down the rendering process)?

I think it does most of the variables once per frame, some things per view or object if there is a material event.

Dont bother worrying, even if the engine pases in those variables, this is very fast as it is not a hardware state change... so basically it wont make any difference either way.

Something you should avoid however is constantly chnageing the texture in a material event, like if you render 50 trees, you shouldnt chnage the material texture for each one in a material event.. this is a state chnage and can be costly if done enough times.

Ah ok, thanks for the info. I'm starting to get the hang of it..