Customer service is king in our business. Offering lots of options has a very real positive affect on how you are viewed. I have had a couple of people who did not like the holster for some reason and took it back gladly... they ended up getting something else and generally some additional items and became happy customers.
So in the end, you need to determine if what the customer is asking will cause your product not to function safely. If it won't be safe, I won't sell/make it.
I have had a time or too where thinking, "Why in the heck would they want to do that!" I end up making it for them and they love it but I would never be able to resell it without modification. If it is one that won't be able to be resold, I make it with a no return or 1/2 price for restocking fee.
Good example for me is one customer that I have made several holsters for with out exact forms for. I gather all the dimensions of the gun, check my forms inventory and see is something is close. If not, look around the house to make my own forms for it. I have another order coming from this customer which is #5 so I can't complain