I see no problem in starting her on a joint supplement just as a precautionary measure.
My parents are a little older and I remember when they started taking glucosamine, they were always talking about how they hadn't realized just how sore their joints had been until they got on the supplement. Maybe that's true for horses too, I don't know.
If you want recommendations, this is my+my hoof trimmers favorite of the supplements Lacey has been on. Lacey has a tendency to drag one of her back toes on the ground while she walks due to some issues in her hock/stifle/fetlock/somewhere in there. Anyway, my trimmer is always commenting to me about it but this last trim (the first one since she's been on this specific supplement, 2 months after Lacey started this supp.) she commented to me that there was no evidence that Lacey had been dragging her toe since the last trim! Since this is the only supplement that has happened for, this is my favorite for now: Corta-FLX Flex-Force Pellets with HA - Horse.com
Talk to your vet as well, maybe he/she will have some even better options for you.