when we bought TJ i was concerned about her weight and so we sought out the advice of an equine nutritionist, that happens to work at our local co-op.
he is also the consultant for the next county over's animal control dept. he's the one that goes out to check on horses when a call has come in regarding their health and safety. he makes the determination whether the horses should be removed from the owners.
we also visited him at length regarding salt blocks vs mineral blocks. his opinion is that if you have a "trace mineral block" available 24/7 for the horse, that is all that is needed. but it must be a "trace mineral" not just a "mineral block". he said it is ok to have both, but not necessary as the trace mineral block has adequate salt in it.
it is just one persons opinion, but that is what we have done.