I dunno about permission, per se. if I feel like I've got something to tell someone that fits better in a personal message, I'd probably do it. but in general I don't really pm/dm with anyone. and nobody really does with me (at least not for long. on occasion, I'll get some rando but even that's really rare).
so I guess my overall feeling is permission is definitely a good thing, but not a hard requirement. esp if it's something you feel needs more urgent attention (hey. did you really mean to post what seems to be your tag number at http://similarworlds/something.blah?)