The Military uses of the Palantiri.

[Inziladun]

Originally Posted by Legate of Amon Lanc

But still, see - for example Aragorn was not viewing anything, he was just "talking" to Sauron, and still it was enough for him. Okay, of course Sauron is Sauron, but I think it was not only his presence or the struggle that ruined Aragorn so much. Does it say anywhere explicitely that there would not be much strain by using them? (Even some strain is enough, though, for you to think it over twice whether to call your cousin in Gondor to just show him how nice lunch you just made for yourself.)

As usual concerning the Palantíri, the best resource seems to be the UT essay.

From Footnote 5:

But when another mind occupied a Stone in accord, thought could be "transferred" (received as "speech"), and visions of the things in the mind of the surveyor of one Stone could be seen by the other surveyor. These powers were originally used mainly in consultation, for the purpose of exchanging news necessary to government, or advice and opinions; less often in simple friendship and pleasure or in greetings and condolance.

So the Stones' primary use was for 'official' purposes, but communicating thoughts of relatively trivial matters with them was not unknown.

As for the general 'strain' of using them, I haven't been able to find a reference specifically for the communications aspect. It does say that the Stones were most easily used by 'Heirs of Isildur', and to a lesser extent, those with legitimate inherited authority in the Realms in Exile.