Does the type of sugar make a difference when making chocolate?
Speaking from making fudge, I have gotten the most consistent results using superfine / castor sugar. 


I haven't noticed starch related issues from powdered sugar, but your mileage may vary; the problem I have had with it is due to clumping in spite of the starch (particularly in a frosting recently). 
I have used granulated sugar to make fudge and it is grainy. I melted most of it, but got sick of stirring it as it got crazy thick. I suppose it's possible to get it to the point of a completely dissolved syrup, but that would take longer than it is worth since you are likely going to run the risk of scorching.


Relative to the taste and structure, you won't get anything special from the caster sugar. However, you will get a more predictable product, and less hassle waiting for it to dissolve. You will also avoid texture problems and reduce the risks brought on by extended time in the pot (i.e. too much heat, likelier to accumulate moisture randomly).