I am not against people putting packs on their dogs; I definitely plan to, but I've been doing a lot of research about this and it's important to determine what your dog can reasonably carry.
First off, if your dog is still growing, don't put a pack on them, or you could hurt their growth plates. I see some people disagreeing with this, but I think it's best to wait 12 to 18 months for most dogs.
Next, don't overload your dogs (25% of their weight), and try to get the weight above the shoulders as much as possible. You can use a pack without weights for sure then slowly add weight using a correctly made pack (lots that aren't). Keep the weight at the front. Always.
Lastly, you need to ensure that your weight ratios are correct before you do any mobility work with your dogs (helping you up if you fall, etc.) I don't know much about training dogs for this kind of Mobility Assistance (and I'm pretty sure you have to be certified to really do this well), but I'm sure that you need to be careful of weight ratios.
There's a lot more to it, but for those also learning about how to pack weights for your dog I think these are good first steps to learn.
If you guys have any other tips, please advise. I'm looking to really be thorough about this!