Sheaths have been a HUGE issue for me since I started making them.
Some customers are more interested in waiting for a knife and getting a sheath of their choice, while others want their knife delivered the day before they bought it.
I'm currently catering to the user market, and with custom fixed blades, I feel a sheath is particularly important. With guns, every gun of a particular model will fit a given sheath, although a custom will still fit slightly nicer. With custom knives, there are no "cookie cutter" sheaths, other than maybe loose fit leather, and even then you'd need at least a few patterns to keep things kosher.
I feel my leather and kydex is on par with the bulk of the sheath making community. I don't claim to be a wizard, but customer feedback on my work has been at least as positive as it has for my knives.
The problem is, I'm a one man shop, full time maker. I don't have time (at all) to drop everything as each knife is sold, and this invariably means customers are going to be left waiting and unhappy. Sometimes I'm too slow, other times the customer is too fast. A lot of the time, I find balance.
I have my schedule tightly regimented, weekends for shop cleaning, design work/planning and sheaths, weekdays for all other aspects of business. I write rough quarterly and detailed monthly schedules, as it is the only way I have found so far to keep things working smoothly for me personally. So if a knife is sold at the end of the week, it might be the following weekend before I can work on it. If anything went wrong during the week, even to the tune of a couple hours, then it could potentially be pushed another week.
My solution so far has been gradual price increases and less choice in the sheath, plus warnings that there will be a wait. It is a big enough liability to my customer service that I am considering just canning sheath work altogether, which is a shame. I do enjoy it, but not in its current state.