I used to use the weight tape to track weight, and I do think it's an ok tool (if that's all you've got!) but if you ever get the chance to bring your horse to the vet instead of having them come out, they almost always have a scale in the clinic that you can use to get a baseline once at least.
The weight tapes are nowhere near accurate on any of my three (two Morgans and a TB). Super over-estimated the Morgans, super under-estimated the TB. But the actual weight taken at the vet + weight tape gave me an idea of how far off (and in which direction) it was. But I mainly gauge weight gain/loss via body condition scores.