Well it depends on who is your buyer. A lady at work who has easily bought 10 cigar pens, likes a bulge in the middle. I made one b2b for her once and she didn't care for it much and gave as a gift to someone else. I do a lot of my current turnings b2b, unless the customer requests otherwise. On any custom pens I make, I ask them, do you want it straight or a bulge in the middle. On my slims I normally at least have a slight hump in the lower barrel as I think it fits in most peoples hands better that way. I have on occasion had requests for a slim to be as straight as possible, but most seem to like the slightly larger middle on the lower barrels. That just has been my experience. Now for my stock I try to have variety, but mostly do b2b. Hope that helps some. In the end it matters only what the customer likes.
Of course I am sure there will be as many opinions on this topic as there are on how & what finish to apply.