G,
It's better to keep some tension for sail control, uniform drum line wrapping & for structural reasons. Remember, the larger the headsail, the more control you want to use. I hate to let a sail get away from me. I use one hand to control the line peeling out while my other hand is winching the sheet(line). Now you have two forms of sail taming. I have a 155 ginney.
If the furling line is allowed to run free you have the possibility of a wrap getting trapped out of sync below top wraps. Ever have a fishing reel free-spin on you & "nest" up? Remember what it was like to work the line backwards to get to the trapped loop? I've seen this happen on some boats where the furling line was not controlled. Now you have the problem where you can't furl it up. I've had to use a screwdriver and work the wraps backwards in the drum until I get to trapped one. In wind, this can be a REAL joy.
In addition, allowing the line to free spin, when the sail hits at all the way out, the bang you hear has a shock chord effect on your forestay & attachment points. When showing beginners the ropes, I call this my big bang theory & it's a no-no.
CR