Barb, for knits I often sew "hem tape" to the edge of the pant leg, then turn up the hem and hand sew the tape to the fabric. There are two types I use. One is solid and you could probably just use the grosgrain ribbon you mentioned. And the other is a lace. I prefer the lace, because if the pants are not straight legged, but a bit flared, it is very easy to take a tiny tuck in the lace edge wherever needed. You can do it with the solid as well, lace is just my preference. You can buy these at Joann and WalMart and lots of other places.

If you are cutting them off you'll have to do something to the cut edge to keep it from raveling. I too would use hem tape and be careful not to stretch the pant edge while sewing it on. If the pant is tapered you may have to let out the side seams a bit and then reinforce the seam. Sew the tape over the gap in the seam. If there is a lot of stretch I'd baste the hem tape on first. Consult your machine manual for sewing stretchy fabrics for the right needle, foot and machine settings. If you don't have a manual I'd use a zig zag stitch and bubble the fabric slightly in front of the needle. Not too much or it will be too tight. Check that it lays flat after you've sewed a bit.

You want a stretchy hem, not a rigid one.

I use stretch lace or a bias tape, sewn on with a zig-zag or stretch stitch (or by hand), and hand hem with a "catch stitch" for a bit of stretch in the hem.

Look at the catch stitch here:

https://blog.colettehq.com/tutorials/5-ways-to-hand-stitch-a-hems 

Yes, if they're dressy pants I would do them with a catch stitch. More casual knit pants would be done with a coverstitch machine.