Dave,
I think thats a good decision, particularly if cost is an issue, as it is for most people. When upgrading I have always tried to buy the items which will get the most use 1st. I would buy the 6mm then the 5mm then the 4mm.
Although my wife considers it all wasted money anyway and sees a new computerised embroidery sewing machine sitting in my eyepiece case every time I open it
CS-John B