I have shot both square and round patches as well as cutting my patches at the muzzle. To tell the truth, I haven't noticed any difference at all so long as your ball is centered on the pre-cut patches. Of course, cutting at the muzzle, when done right, will guarantee that your ball is centered.
When I pre-cut my patches, whether round or square, I make them approximately 2 1/2 to 3 times the diameter of the ball. This insures that the patch will cover the ball and contact the lands and grooves with no extra material covering the ball. I don't know that having too big of a patch so that it wraps completely around the ball and folds over is a bad thing but I just do not want to have that much material to possibly interfere with the ball after it leaves the barrel. I don't know that too much patch material actually hurts anything but I choose not to have it in the way. Besides, it does not seem to be beneficial in any way to have too much patch. I can tell you from experience that if you use a short starter, as many people do, and it has a close fitting tip that is near the bore size, having too large of a patch can cause the patch material to wrap onto the end of your short starter and grab it. This can result in having a tug of war with your short starter to get it out of the muzzle. :thumbsup:
These are just the thoughts of an old fart who has been building and shooting muzzleloaders for over 40 years. The opinions are free and, no doubt, over priced at that. :haha: