It seems to me that it would be very simple to see if the shop was right. Install a tubeless peep and see if it moves. This will be inexpensive to do and the peep can still be used if you decide to go with the better string. You can get custom made strings a lot cheaper, but a good Winners Choice String will cost in that ballpark.

Personally, I'm one who believes the string is probably more important than the bow you choose. Even though the bowshop person did not come across well, he may be right.