Change screws if you need to reduce weight, otherwise it's not that big a deal.
The priority is rotating weight. Anything that can be done to reduce the weight of the drivetrain translates to better responsiveness to the throttle. However, making it weaker is not a good idea!
Before anything is changed, assess its loads. Where there are shear loads (eg stand offs) then steel is your best option. Also, where you need high loads (eg grubscrews on shafts) steel is also the material of choice.
Compressive/tensile loads (eg shock towers or chassis decks) can probably take an aluminium screw, but there is also the possibility of a shear load in an accident. If it's a screw holding a servo to a post, aluminium will probably do it. A steering rod is great for titanium. However, a shock mount bolt takes shear and tensile loads, so steel is probably best.
Think application first, then think weight. Considering both, then choose the right material for the job. There is no simple answer, just a good engineering solution. HTH