Rules for the message / vision / prophecy to send to the past
A first - unavoidable - step in using time travel technology is testing it, to confirm that it works and that messages actually can be send. A message that could serve as proof must be rather special. Only a message that without doubt confirms that it could only come from a future moment in time would qualify and would be able to give such confirmation. A true prophecy would be the perfect mysterious message that would fit the bill. Although such a message should be an acurate description of a future event, it should not cause any alteration of the past as that could have severe consequences for the course of history.
Ideally a prediction will only be recognized as a prediction after the event, requiring careful planning and coding in order to avoid premature discovery.
It might turn out to be impossible to send a message to the past without altering history. The question then might be as to what level of alteration caused by time travel is still acceptable.
To be useful, a prophet must record the message as received. It might happen however that in interpreting the vision it gets distorted (Chinese whispers effect). In such case the prophet's work might not be helpful at all to prove any time travel experiment. Therefore it is imperative that the vision to send must be clear and understandable.
If a prophet only makes a single prediction during his life then it would be no problem if that one prediction would be highly accurate and open. The probability that his work would be taken serious, analyzed and reacted upon before the event is small.
However the amount of information that scientists need back from a prophet in order to proof a link between their experiment and his work of prophecy would probably need many more messages with acurate prophecies. Since hackers will lurk on unfulfilled prophecies of known prophets, consecutive visions need to be concealed and coded much better.
If the work of a prophet gains too much attention it might cause adverse effects. The seers could gain so much respect that people would see him as a (religious) prophet and his work might start a religion with possibly many followers. The start of such following would definitely alter the course of history and should be avoided.
Since the existence of mayor events is easier to prove, it seems better to stick to sending visions about such events. Although some of these mayor events might refer to natural disasters, great care should be taken that the natural disaster chosen cannot be calculated and predicted by other means as well.