Hi guys. There seems to be alot of questions about roping lately huh. I think there are two main things that cause roping. I have changed my view somewhat in the last few months, the chainstitching does help but it is very minor, and will help more on unsanforized denim. The nice thick stitching on the hem when cuffed does look very nice.
The two main things that will cause roping in my opinion are, how the hem is rolled and the width of the hem.
The old union special 4G.. what ever it is, and any other older, newer, other brand of chain stitcher that has a down turn hemmer attachment will roll the hem crooked. If you cuff your jeans and look at the seam of the selvage, then the seam of the hem it will be slightly off centre. If it is really twisted so the hem seam is nearly touching the white line of the selvage, you will probably get more roping in the long run.
The width of the hem is important also, if it is too small it won't rope up as good and if it is too big, 2cm'ish it won't either. On some of the brands mentioned with the larger hem...the machine is similar, but it turns the hem much neater, seams exactly straight, 2cm width and with a lock stitch. These are like a neat trouser hem. Pic below...Union Special brand.
If you are going to the tailor to get your jeans hemmed, just get them to sewn it crooked at a 1cm hem width. Here is a picture below I nicked from somewhere to show the crooked'ness.