for right handed golfers,
on impact when your weight is shifted to your left leg and your hip is moved outward towards the left, should your left leg be flexed or straight?
Even Tiger Woods has his left leg slightly flexed at impact, and he's the one who popularized really getting that left knee straight. I remember a few times, announcers would analyze his swing, and show how he snaps that knee straight just after impact. But the left leg does have some bend into it, the amount of bend depends on the player.
honestly, most amateurs need more flex in there left leg to promote getting that weight forward at impact. If your straightening it, its most likely you have yuor weight on your back food, and are straightening the left leg just because that's what it looks like at the finish. Its like getting to the finish line way to early.