Greasy Mushrooms?
 

Putting the front uprights together and I've come accross an interesting dillema:

Should I grease the mushrooms before insertion?

Similar for the ball-joint that goes in the mushroom; greasy ball? ;)

Any advice appreciated

As you don't want them to move once assembled (with the exception of the ball in the ball joint) I'd say no, don't grease.

That answers that question.

I was leaning that way anyway, but wasn't sure if it might help as a barrier between the wetals (ali on steel).

On a similair theme, any advice on mushroom placement?
I know the holes are meant to be forwards, but how accurate do they need to be? Is there an easy way to measure/allign them?

If you're worried about galvanic corrosion of the aluminium from the cast iron, paint both surfaces with duralac.

We use it at work between stainless and mild.

Aluminium shouldn't be too much of a problem though as it forms an oxide layer really quickly which prevents most corrosion outside of a marine environment.

Just visually forwards (take the surface of the wheel/brake disk as the definition of "forwards") is fine. Any sideways imperfection will be corrected when setting camber. When you're done, put a notch in the mushroom and upright, that will help if you have to realign them later.