i have been reading up on nib tuning. It really is an interesting subject, and sounds a bit of an art. I have the sum experience of playing with one nib, although I do a lot of writing with a fountain pen, and have done for years. Like wine, I know what I like! Take the following as a collection of my thoughts rather than speaking from any real experience.
I made a prototype pen, with a bock #5 nib in. I think that it was a fine- it was certainly finer than either of my 2 usual pens, which are medium-ish. I found it to be ok in use, but certainly not as wet as my usual users. My daughter Madeleine then used it, but being new to fountain pens, not quite as adaptive with changing writing angle to suit the pen, and left handed, she got some different results. She is 5 1/2, but wanted to try it out, so I was happy to encourage her, as she will be expected to use one at school later this year. The results that she got were a bit more that it worked or it didnt work and skipped. I turned the pen so that i was looking at the back of the nib, and put the side of my thumb nails on each side of the nib, and gently pushed back. This had the effect of opening the slit a fraction, and we both felt that it wrote better being "wetter". She wrote nicely with it, it didnt skip and on this fine nib, there was no need for a left handed version.
I am going to make her her own pen next, and will certainly do a bit more tuning to the nib than I did with this- although I will say that the bocks seem to be good quality from the get go. The problem is that a good nib is only good if it suits the user, if it doesnt, it can be tweeked. If it were for myself, I would probably not do much more than open the slit a little if it needed it. Madeleine holds the pen at a slightly steeper angle than I do, so making a few dozen figure of eights on micromesh at her preferred angle would take the hint of scratch out of it. I would do this for her because 1. I know the end user and what needs to be achieved, and 2. the work wont be chargable!!
I dont sell many pens, although I plan to sell more in the future, so I try to keep a commercial eye on things. I wouldnt want to spend long tuning a poor nub- having to regrind and shape a cheap and nasty one would be false economy when you can buy a bock from Phil for £8 or so, which is better quality, branded and doesnt have many of the problems that need correcting on the cheapie. That £8 doesnt buy much workshop time spent fiddling. This should be for refining, and tailoring, rather than correcting (in my humble opinion).
I dont know how well prepared a £50 cross/shaefer/parker fountain pen is straight from WH Smiths but what you sell has to be fit for purpose and write nicely. I had a cheapish shaefer when I was at school, and it was a nice writer. I have no idea why, but I bought it, used it and it was nice. You will be judged on how the pen writes the first time it is used- smooth and you may get a repeat customer, scratchy and the pen may never write another page.
Some of the early pens I made were 2 Omegas for a good friend in the USA. I would very much like to have a look at them now and spend a few minutes test driving them/applying what I have read. I dont ink pens before giving them to the recipient, and didnt know much about nibs when I made them. She seems happy, but I think about that one I made (described above), and how it improved with the minimal amount of tuning that I did.