Hi Phil...i have made a no of fountain pens and most people are happy with the standard nibs,either there not aware they can be changed or not that fussy.
However i have had the odd request different nibs, to this i went to Craft Supplies and bought 2 of each no5 and no6 in ef/f/m and b this has helped and i find they are easy to change.It would be a good idea to have easy access to replacement nibs but i think this may be a slow seller for you.
Hope this helps